Unusual red olive is a really bad one (8)
I believe the answer is:
evildoer
'a really bad one' is the definition.
(a bad person)
'unusual red olive' is the wordplay.
'unusual' is an anagram indicator.
'red'+'olive'='redolive'
'redolive' anagrammed gives 'EVILDOER'.
'is' acts as a link.
(Other definitions for evildoer that I've seen before include "sinner" , "Rode live over morally bad person" , "One who commits very bad deeds" , "Wicked person" , "'baddie'?" .)