Ashes of the French saint (4)
I believe the answer is:
dust
'ashes' is the definition.
(thesaurus)
'of the french saint' is the wordplay.
'of the french' becomes 'du' ('of the' in French).
'saint' becomes 'st' (abbreviation).
'du'+'st'='DUST'
(Other definitions for dust that I've seen before include "Unwanted fine powder" , "The mortal remains" , "Fine powdery material that can be blown about in the air" , "Tiny particles of dirt, skin flakes etc" , "Stud (anag)" .)