Dog beginning to bother bird (6)
I believe the answer is:
beagle
'dog' is the definition.
(I know that Beagle is a type of dog)
'beginning to bother bird' is the wordplay.
'beginning to' suggests taking the first letters.
'bird' becomes 'eagle' (eagle is a type of bird).
The initial letter of 'bother' is 'b'.
'b'+'eagle'='BEAGLE'
(Other definitions for beagle that I've seen before include "Darwin voyaged in this ship" , "Hunting dog followed on foot" , "Breed of dog in a glebe" , "hunting animal with smooth coat" , "Short-haired dog" .)