Remain a saint or a monster! (5)
I believe the answer is:
beast
'a monster' is the definition.
(I've seen this before)
'remain a saint' is the wordplay.
'remain' becomes 'be' (I've seen this in other clues).
'saint' becomes 'st'.
'be'+'a'+'st'='BEAST'
'or' is the link.
(Other definitions for beast that I've seen before include "Bully" , "Traditional poet" , "Animal; cruel person" , "beef?" , "wild creature" .)