A French word returned is a particle (4)
I believe the answer is:
atom
'particle' is the definition.
(synonyms)
'a french word returned' is the wordplay.
'french word' becomes 'mot' (I've seen this in other clues).
'returned' says the letters should be written backwards.
'mot' back-to-front is 'tom'.
'a'+'tom'='ATOM'
'is a' acts as a link.
(Other definitions for atom that I've seen before include "Elementary particle" , "Something very small perhaps" , "A mighty particle, makes huge explosion" , "Tiny particle, causes mighty explosion" , "A small but mighty particle" .)