A bird left Noah's boat (4)
I believe the answer is:
lark
'a bird' is the definition.
(lark is a kind of bird)
'left noah's boat' is the wordplay.
'left' becomes 'l' (common abbreviation).
'noah's boat' becomes 'ark' (I've seen this before).
'l'+'ark'='LARK'
(Other definitions for lark that I've seen before include "flier" , "Small songbird or playful action" , "High-flying singer - frolic" , "Bird - caper" , "Songbird - 3" .)