Clip tail of aggressive bird (5)
I believe the answer is:
snipe
'bird' is the definition.
(snipe is a kind of bird)
'clip tail of aggressive' is the wordplay.
'clip' becomes 'snip' ('snip' can be a synonym of 'clip').
'tail of' says to take the final letters.
The final letter of 'aggressive' is 'e'.
'snip'+'e'='SNIPE'
(Other definitions for snipe that I've seen before include "To pick off soldiers, say" , "The bird to shoot from cover" , "From concealed position, shoot" , "Wading bird with long straight bill" , "Perhaps carp" .)