Initially breeding bird and dog (6)
I believe the answer is:
beagle
'dog' is the definition.
(I know that beagle is a type of dog)
'initially breeding bird' is the wordplay.
'initially' indicates taking the first letters.
'bird' becomes 'eagle' (eagle is a type of bird).
The first letter of 'breeding' is 'b'.
'b'+'eagle'='BEAGLE'
'and' is the link.
(Other definitions for beagle that I've seen before include "Small short-legged hound" , "hunting animal with smooth coat" , "Darwin voyaged in this ship" , "Small hunting dog" , "Small, sturdy breed of hound" .)