A bird or part of a bird (7)
I believe the answer is:
bittern
'bird' is the definition.
(bittern is a kind of bird)
'a bird or part of a' is the wordplay.
I cannot really understand how this works, but
'bird' could be 'tern' (tern is a kind of bird) and 'tern' is located in the answer.
'part' could be 'bit' (both mean a piece of something) and 'bit' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bittern that I've seen before include "Aquatic bird" , "Bird that booms" , "Marshland bird" , "Wading bird" , "flier" .)