Large boat’s bird (4)
I believe the answer is:
lark
'bird' is the definition.
(lark is a kind of bird)
'large boat's' is the wordplay.
'large' becomes 'l'.
'boat' becomes 'ark' (ark is a kind of boat).
'l'+'ark'='LARK'
(Other definitions for lark that I've seen before include "Songbird - 3" , "Rise with this bird to get up early" , "One in exaltation" , "Songbird; escapade" , "High-flying songbird" .)