Impertinence that is concerning supporter (9)
I believe the answer is:
brassiere
'supporter' is the definition.
(a bra gives support to breasts)
'impertinence that is concerning' is the wordplay.
'impertinence' becomes 'brass' (I've seen this before).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'concerning' becomes 're' (both can mean on a particular topic).
'brass'+'ie'+'re'='BRASSIERE'
(Other definitions for brassiere that I've seen before include "support" , "Female undergarment supporter" , "garment" .)