Second-class bird dog? (6)
I believe the answer is:
beagle
'dog?' is the definition.
(I know that beagle is a type of dog)
'second-class bird' is the wordplay.
'second class' becomes 'b'.
'bird' becomes 'eagle' (eagle is a type of bird).
'b'+'eagle'='BEAGLE'
(Other definitions for beagle that I've seen before include "Hunting hound" , "Short-haired dog" , "Hound dog" , "Small, sturdy breed of hound" , "Type of hound" .)