B-bird dog (6)
I believe the answer is:
beagle
'dog' is the definition.
(I know that beagle is a dog breed)
'b-bird' is the wordplay.
'bird' becomes 'eagle' (eagle is a type of bird).
'b'+'eagle'='BEAGLE'
(Other definitions for beagle that I've seen before include "Breed of dog in a glebe" , "Small short-legged hound" , "5 [MANS BEST FRIEND]" , "hunting animal with smooth coat" , "HMS - was Darwin's ship; hunting dog" .)