Reveal the French put on weight (3,2)
I believe the answer is:
let on
'reveal' is the definition.
(synonyms)
'the french put on weight' is the wordplay.
'the french' becomes 'le' ('the' in French).
'put on' is a charade indicator (letters next to each other).
'weight' becomes 'ton' (unit of weight).
'le'+'ton'='LET ON'
(Other definitions for let on that I've seen before include "Divulge information" , "Be revealing" , "Divulge (secret to)" , "Permitted to board" , "Divulge info; pretend" .)