Bird breeds with rat, remarkably
I believe the answer is:
redbreast
'bird' is the definition.
(redbreast is a kind of bird)
'breeds with rat remarkably' is the wordplay.
'with' means one lot of letters go next to another.
'remarkably' is an anagram indicator.
'breeds' put after 'rat' is 'ratbreeds'.
'ratbreeds' with letters rearranged gives 'REDBREAST'.
(Other definitions for redbreast that I've seen before include "Type of robin" , "One tweets" , "Bird" , "Flapper" .)