Birds - little birds (8)
I believe the answer is:
bitterns
'birds' is the definition.
(bittern is a kind of bird)
'little birds' is the wordplay.
'little' becomes 'bit' (a little is a bit).
'birds' becomes 'terns' (tern is a kind of bird).
'bit'+'terns'='BITTERNS'
(Other definitions for bitterns that I've seen before include "Bert isn't involved with heron-like birds" , "Waders" , "Marsh birds" , "Small herons with loud cries" .)