Island; linear measure (4)
I believe the answer is:
inch
I believe this clue is a double definition.
'island' is the first definition.
(inch can mean a small island)
'linear measure' is the second definition.
(inch is a kind of linear measure)
(Other definitions for inch that I've seen before include "The island is very small in length" , "... or not so long < , Celtic island" , "Move in tiny increments" , "One of a 2 down in a foot" , "A very little" .)