A French word returned is a particle (4)
I believe the answer is:
'particle' is the definition.
'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'.
'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" .)