Bird left vessel (4)
I believe the answer is:
'bird' is the definition.
(lark is a kind of bird)
'left vessel' is the wordplay.
'left' becomes 'l' (common abbreviation).
'vessel' becomes 'ark' (ark is a kind of vessel).
(Other definitions for lark that I've seen before include "Amusing escapade; bird" , "bit of mischief" , "what one may get up to?" , "Bird; prank" , "flier" .)