The flier left the vessel (4)
I believe the answer is:
lark
'the flier' is the definition.
(lark is a kind of birdanimal)
'left the vessel' is the wordplay.
'left' becomes 'l' (common abbreviation).
'the vessel' becomes 'ark' (ark is a kind of vessel).
'l'+'ark'='LARK'
(Other definitions for lark that I've seen before include "One singing high" , "One gets up early" , "bit of mischief" , "Amusing adventure; bird" , "Bird that sings on the wing" .)