One bird pecked another (7)
I believe the answer is:
bittern
'one bird' is the definition.
(bittern is a kind of bird)
'pecked another' is the wordplay.
'pecked' becomes 'bit' (I've seen this in other clues).
'another' becomes 'tern' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bit'+'tern'='BITTERN'
Can you help me to learn more?
(Other definitions for bittern that I've seen before include "Marshland bird" , "Wading bird" , "Bird that booms" , "flier" , "Compact tawny-brown heron with nocturnal habits and a booming cry" .)