Bewitched at start, bird becomes dog (6)
I believe the answer is:
beagle
'dog' is the definition.
(I know that beagle is a type of dog)
'bewitched at start bird' is the wordplay.
'at start' indicates taking the first letters.
'bird' becomes 'eagle' (eagle is a kind of bird).
The first letter of 'bewitched' is 'b'.
'b'+'eagle'='BEAGLE'
'becomes' is the link.
(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" , "Small short-legged hound" , "Hunting hound" .)