Bird breeds with rat, bizarrely (9)
I believe the answer is:
redbreast
'bird' is the definition.
(redbreast is a kind of bird)
'breeds with rat bizarrely' is the wordplay.
'with' says to put letters next to each other.
'bizarrely' indicates anagramming the letters.
'breeds' after 'rat' is 'ratbreeds'.
'ratbreeds' with letters rearranged gives 'REDBREAST'.
(Other definitions for redbreast that I've seen before include "Flapper" , "Type of robin" , "Bird" , "One tweets" .)