Bird nibbled beside another one (7)
I believe the answer is:
bittern
'bird' is the definition.
(bittern is a kind of bird)
'nibbled beside another one' is the wordplay.
'nibbled' becomes 'bit' (I've seen this in other clues).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'another one' becomes 'tern' (I can't justify this - if you can you should give a lot more credence to this answer).
'bit'+'tern'='BITTERN'
(Other definitions for bittern that I've seen before include "Bird with a booming cry" , "found hidden in reed-beds" , "'He shall not hear the . . . cry' (Ledwidge)" , "Aquatic bird" , "inhabitant of marsh" .)