Bird's loud -- a large rook (6)
I believe the answer is:
falcon
'bird's' is the definition.
(falcon is a kind of bird)
'loud a large rook' is the wordplay.
'loud' becomes 'f' (A musical term).
'large' becomes 'l'.
'rook' becomes 'con' ('con' can be a synonym of 'rook').
'f'+'a'+'l'+'con'='FALCON'
(Other definitions for falcon that I've seen before include "See 16" , "Kind of hunting bird - peregrine is one" , "A hunting bird, may be peregine" , "Diving bird of prey" , "Long-winged hawk" .)