Small ship for small island (4)
I believe the answer is:
sark
'small island' is the definition.
(I've seen this before)
'small ship' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ship' becomes 'ark' (as in Noah's ark).
's'+'ark'='SARK'
'for' acts as a link.
(Other definitions for sark that I've seen before include "Hereditarily-ruled Channel Island" , "Small Channel Island" , "English Channel isle" , "See 4 Down" , "Shirt (Sc.); Ch. island" .)