One bird pecked another bird (7)
I believe the answer is:
bittern
'one bird' is the definition.
(bittern is a kind of bird)
'pecked another bird' is the wordplay.
'pecked' becomes 'bit' (I've seen this in other clues).
'another bird' becomes 'tern' (tern is a kind of bird. I am not sure about the 'another' bit.).
'bit'+'tern'='BITTERN'
Can you help me to learn more?
(Other definitions for bittern that I've seen before include "Marshland bird" , "Marsh bird of the heron family" , "flier" , "inhabitant of marsh" , "'He shall not hear the . . . cry' (Ledwidge)" .)