Large bird goes after black dog (6)
I believe the answer is:
beagle
'dog' is the definition.
(beagle is a kind of dog)
'large bird goes after black' is the wordplay.
'large bird' becomes 'eagle' (I've seen this before).
'goes after' says to put letters next to each other.
'black' becomes 'b' (as in pencil classifications).
'eagle' put after 'b' is 'BEAGLE'.
(Other definitions for beagle that I've seen before include "Small hunting dog" , "hunting animal with smooth coat" , "Hunting dog followed on foot" , "Hunting hound" , "Small short-legged hound" .)