Rebel Frenchman aged badly (8)
I believe the answer is:
renegade
'rebel' is the definition.
(I've seen this before)
'frenchman aged badly' is the wordplay.
'frenchman' becomes 'rene'.
'badly' is an anagram indicator.
'aged' anagrammed gives 'gade'.
'rene'+'gade'='RENEGADE'
(Other definitions for renegade that I've seen before include "Dare Gene to become a rebel" , "Treacherous person" , "Apostate" , "disloyal type" , "Traitor" .)