Put up with the beast! (4)
I believe the answer is:
bear
I believe this clue is a double definition.
'put up with' is the first definition.
('bear' can be a synonym of 'put up with')
'the beast' is the second definition.
(bear is a kind of beast)
(Other definitions for bear that I've seen before include "rude fellow" , "Put up with - beast!" , "Ursine animal" , "Support; share seller" , "stomach and > suffer" .)