Supporter of pelota's turning nasty (7)
I believe the answer is:
apostle
'supporter' is the definition.
(apostle is a kind of supporter)
'pelota's turning nasty' is the wordplay.
'turning nasty' indicates anagramming the letters (letters corrupted or damaged).
'pelotas' with letters rearranged gives 'APOSTLE'.
'of' is the link.
(Other definitions for apostle that I've seen before include "Good man" , "Luke or John, say" , "Augustine, say" , "[JAMES] , for one?" , "One of Christ's 12 original disciples" .)