Information:  - Clett is a small uninhabited Scottish islet near the north coast of Caithness , just to the west Holborn Head north of Scrabster .  - Old Norse was a North Germanic language that was spoken by inhabitants of Scandinavia and inhabitants of their overseas settlements during about the 9th to 13th centuries.  - Thurso (pronounced , ) is a town and former burgh on the north coast of the Highland council area of Scotland. Situated in the historical area of Caithness, it is the northernmost town on the British mainland. It lies at the junction of the north-south A9 road and the west-east A836 road, connected to Bridge of Forss in the west and Castletown in the east. The River Thurso flows through the town and into Thurso Bay and the Pentland Firth. The river estuary serves as a small harbour. At the 2011 Census, Thurso had a population of 7,933. The larger Thurso civil parish including the town and the surrounding countryside had a population of 9,112.  - Scotland (Scots: ) is a country that is part of the United Kingdom and covers the northern third of the island of Great Britain. It shares a border with England to the south, and is otherwise surrounded by the Atlantic Ocean, with the North Sea to the east and the North Channel and Irish Sea to the south-west. In addition to the mainland, the country is made up of more than 790 islands, including the Northern Isles and the Hebrides.  - Caithness (, ; Old Norse: "Katanes") is a historic county, registration county and lieutenancy area of Scotland.  - Thurso Bay, known also as Scrabster Bay, is a bay of Atlantic water between the points of Clairdon Head and Holborn Head on the north coast of Caithness, Scotland.  - A promontory fort is a defensive structure located above a steep cliff, often only connected to the mainland by a small neck of land, thus utilizing the topography to reduce the ramparts needed. Although their dating is problematic, most seem to date to the Iron Age. They are mainly found in Brittany, Ireland, the Orkney Islands, the Isle of Man, Devon and Cornwall.  - Inverness (from the , meaning "Mouth of the River Ness") is a city in the Scottish Highlands. It is the administrative centre for the Highland council area, and is regarded as the capital of the Highlands of Scotland. Inverness lies near two important battle sites: the 11th-century battle of Blàr nam Fèinne against Norway which took place on The Aird and the 18th-century Battle of Culloden which took place on Culloden Moor. It is the northernmost city in the United Kingdom and lies within the Great Glen (Gleann Mòr) at its north-eastern extremity where the River Ness enters the Moray Firth. At the latest, a settlement was established by the 6th century with the first royal charter being granted by Dabíd mac Maíl Choluim (King David I) in the 12th century. The Gaelic king Mac Bethad Mac Findláich (MacBeth) whose 11th-century murder of King Duncan was immortalised in Shakespeare's play "Macbeth", held a castle within the city where he ruled as Mormaer of Moray and Ross.  - Scrabster is a small settlement on Thurso Bay in Caithness on the north coast of Scotland. It is some from Thurso, from Wick and from Inverness.  - Holborn Head is a headland on the north-facing Atlantic coast of Caithness, in the Highland area of Scotland. The "point" of Holborn Head is at (grid reference ). The name "Holborn" appears Norse in origin, meaning "hillfort", and the headland may be the "Tarvedunum promotorium" noted by Ptolemy. At the tip lies the remains of a promontory fort.  - A registration county was, in Great Britain and Ireland, a statistical unit used for the registration of births, deaths and marriages and for the output of census information. In Scotland registration counties are used for land registration purposes.    Given the paragraphs above, decide what entity has the relation 'instance of' with 'island'.
clett