Bird leaves loch (4)
I believe the answer is:
teal
'bird' is the definition.
(teal is a kind of bird)
'leaves loch' is the wordplay.
'leaves' becomes 'tea' (I've seen this before).
'loch' becomes 'l'.
'tea'+'l'='TEAL'
(Other definitions for teal that I've seen before include "Duck; greenish-blue colour" , "Type of duck" , "Dark greenish-blue colour" , "Variety of duck" , "Bluish shade of green" .)