Bird leaves lake (4)
I believe the answer is:
teal
'bird' is the definition.
(type of duck)
'leaves lake' is the wordplay.
'leaves' becomes 'tea' (as in tea leaves).
'lake' becomes 'l' (geographical abbreviation).
'tea'+'l'='TEAL'
(Other definitions for teal that I've seen before include "bird" , "Raise (subject matter)" , "Dark greenish-blue colour" , "Variety of duck" , "Variety of small ducks" .)