British bird dog (6)
I believe the answer is:
beagle
'dog' is the definition.
(beagle is a kind of dog)
'british bird' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'bird' becomes 'eagle' (eagle is a type of bird).
'b'+'eagle'='BEAGLE'
(Other definitions for beagle that I've seen before include "Small hunting dog" , "Darwin voyaged in this ship" , "Short-haired dog" , "5 [MANS BEST FRIEND]" , "Breed of dog in a glebe" .)