Ally ousting Republican devil (5)
I believe the answer is:
fiend
'devil' is the definition.
(I know that devil can be written as fiend)
'ally ousting republican' is the wordplay.
'ally' becomes 'friend' (synonyms).
'ousting' is a deletion indicator.
'republican' becomes 'R' (abbreviation).
'friend' with 'r' taken away is 'FIEND'.
(Other definitions for fiend that I've seen before include "Demon; cruel person" , "Demon, ogre" , "Wicked and inhuman person" , "Devilish person" , "monster" .)