Bird initially leaving boat in flood (4)
I believe the answer is:
lark
'bird' is the definition.
(I know this)
'initially leaving boat in flood' is the wordplay.
'initially' suggests taking the first letters.
'boat in flood' becomes 'ark' (Noah's ark).
The initial letter of 'leaving' is 'l'.
'l'+'ark'='LARK'
(Other definitions for lark that I've seen before include "Bird - caper" , "One gets up early" , "Rise with this bird to get up early" , "Songbird - bit of fun" , "jolly occasion" .)