Flash of French fashion (4)
I believe the answer is:
mode
'fashion' is the definition.
('mode' can be a synonym of 'fashion')
'flash of french' is the wordplay.
'flash' becomes 'mo' (I've seen this in other clues).
'of french' becomes 'de'.
'mo'+'de'='MODE'
(Other definitions for mode that I've seen before include "Form or style" , "Style; statistical value" , "Style of dome" , "Fashion; way of performing" , "Way of operation" .)