Audacity namely concerning undergarment (9)
I believe the answer is:
brassiere
'undergarment' is the definition.
(brassiere is a kind of undergarment)
'audacity namely concerning' is the wordplay.
'audacity' becomes 'brass' (I've seen this in another clue).
'namely' becomes 'ie' (short 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 "Female undergarment supporter" , "garment" , "support" .)