French impudence is charming at first (6)
I believe the answer is:
gallic
'french' is the definition.
(I know that french can be written as Gallic)
'impudence is charming at first' is the wordplay.
'impudence' becomes 'gall' (synonyms).
'at first' suggests taking the first letters.
The first letters of 'is charming' is 'ic'.
'gall'+'ic'='GALLIC'
(Other definitions for gallic that I've seen before include "Pertaining to France" , "Typically French or of France" , "Like certain wars" , "Of the French/France" , "Characteristic of the French" .)