Dog initially biting bird of prey (6)
I believe the answer is:
beagle
'dog' is the definition.
(beagle is a kind of dog)
'initially biting bird of prey' is the wordplay.
'initially' indicates taking the first letters.
'bird of prey' becomes 'eagle' (eagle is a kind of bird of prey).
The initial letter of 'biting' is 'b'.
'b'+'eagle'='BEAGLE'
(Other definitions for beagle that I've seen before include "Hound dog" , "Hunting hound" , "Darwin voyaged in this ship" , "5 [MANS BEST FRIEND]" , "HMS - was Darwin's ship; hunting dog" .)