Bird in vessel on lake (4)
I believe the answer is:
lark
'bird' is the definition.
(lark is a kind of bird)
'vessel on lake' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel**).
'on' is a charade indicator (letters next to each other).
'lake' becomes 'l'.
'ark' put after 'l' is 'LARK'.
'in' is the link.
(Other definitions for lark that I've seen before include "Fool" , "jolly occasion" , "One gets up early" , "Bird; prank" , "what one may get up to?" .)