Start breeding a bird dog (6)
I believe the answer is:
beagle
'dog' is the definition.
(I know that beagle is a type of dog)
'start breeding a bird' is the wordplay.
'start' says to take the initial letters.
'a bird' becomes 'eagle' (eagle is a type of bird).
The first letter of 'breeding' is 'b'.
'b'+'eagle'='BEAGLE'
(Other definitions for beagle that I've seen before include "Small, sturdy breed of hound" , "Small short-legged hound" , "Type of hound" , "Hunting hound" , "Breed of dog in a glebe" .)