Oust a rogue turning monstrous (10)
I believe the answer is:
outrageous
'monstrous' is the definition.
(thesaurus)
'oust a rogue turning' is the wordplay.
'turning' is an anagram indicator.
'oust'+'a'+'rogue'='oustarogue'
'oustarogue' anagrammed gives 'OUTRAGEOUS'.
(Other definitions for outrageous that I've seen before include "Exorbitant" , "undignified" , "Shocking" .)