Exposes a French medium with demands (7)
I believe the answer is:
unmasks
'exposes' is the definition.
(unmasking is a kind of exposing)
'a french medium with demands' is the wordplay.
'a french' becomes 'un' ('a' in French).
'medium' becomes 'm' (as in UK clothing sizes).
'with' is a charade indicator (letters next to each other).
'demands' becomes 'asks' ('ask' can be a synonym of 'demand').
'un'+'m'+'asks'='UNMASKS'
(Other definitions for unmasks that I've seen before include "lays oneself open" , "Removes domino" , "Reveals the true nature of" , "Exposes, removes disguise" , "Exposes, shows up" .)