Vessel behind heading for small Channel island (4)
I believe the answer is:
sark
'channel island' is the definition.
(I've seen this before)
'vessel behind heading for small' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel**).
'behind' is a charade indicator (letters next to each other).
'heading for' suggests taking the first letters.
The first letter of 'small' is 's'.
'ark' put after 's' is 'SARK'.
(Other definitions for sark that I've seen before include "One of the Channel Islands" , "Shirt (Sc.); Ch. island" , "See 4 Down" , "One of Channel Isles" , "Island in English Channel" .)