Ruthless rogue, the French guy (10)
I believe the answer is:
implacable
'ruthless' is the definition.
(thesaurus)
'rogue the french guy' is the wordplay.
'rogue' becomes 'imp'.
'the french' becomes 'la' ('the' in French).
'guy' becomes 'cable' (I've seen this in other clues).
'imp'+'la'+'cable'='IMPLACABLE'
(Other definitions for implacable that I've seen before include "Relentless" , "Totally unrelenting" , "Pitiless, unyielding" , "Unwilling to be appeased" , "unable to be stopped" .)