Used car repaired by Romeo for campaigner (8)
I believe the answer is:
crusader
'campaigner' is the definition.
(eg someone on a moral crusade)
'used car repaired by romeo' is the wordplay.
'repaired' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'used'+'car'='usedcar'
'usedcar' anagrammed gives 'crusade'.
'crusade'+'r'='CRUSADER'
'for' acts as a link.
(Other definitions for crusader that I've seen before include "religious zealot" , "religious warrior" , "Richard I, for instance" , "Medieval expeditionary" , "RA cursed (anag) - one fighting for a cause" .)