Heartless devil, the French guy (10)
I believe the answer is:
implacable
'heartless' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'devil the french guy' is the wordplay.
'devil' becomes 'imp' (devilish creature).
'the french' becomes 'la' (one of the French words for 'the').
'guy' becomes 'cable' (I've seen this in other clues).
'imp'+'la'+'cable'='IMPLACABLE'
Can you help me to learn more?
(Other definitions for implacable that I've seen before include "adamant" , "Pitiless, unyielding" , "Totally unrelenting" , "Unforgiving" , "unable to be stopped" .)