Extremities of Sark, the ancient island (4)
I believe the answer is:
skye
'island' is the definition.
(I've seen this before)
'extremities of sark the ancient' is the wordplay.
'extremities of' means to remove the middle letters.
'the ancient' becomes 'ye'.
'sark' with its middle taken out is 'sk'.
'sk'+'ye'='SKYE'
(Other definitions for skye that I've seen before include "Destination of a princely boat trip" , "Portree island" , "Island in a boat song about Flora MacDonald and Bonnie Prince Charlie" , "Large Scottish island" , "Largest island of Scotland's Inner Hebrides" .)