Once impudent, is beginning to exercise savoir-faire (9)
I believe the answer is:
expertise
'savoir-faire' is the definition.
(thesaurus)
'once impudent is beginning to exercise' is the wordplay.
'once' becomes 'ex'.
'impudent' becomes 'pert' (I've seen this before).
'beginning to' says to take the initial letters.
The first letter of 'exercise' is 'e'.
'ex'+'pert'+'is'+'e'='EXPERTISE'
(Other definitions for expertise that I've seen before include "Pert sex, i.e. could become skilled knowledge" , "Technical skill" , "Mastery, proficiency" , "Thorough knowledge" , "Special skill or knowledge" .)