The fall of French currency (7)
I believe the answer is:
descent
'the fall' is the definition.
(synonyms)
'of french currency' is the wordplay.
'of french' becomes 'des' (I've seen this in other clues).
'currency' becomes 'cent' (cent is a kind of currency).
'des'+'cent'='DESCENT'
(Other definitions for descent that I've seen before include "Downward slope" , "line" , "A climb down" , "Fall" , "Drop" .)