Barrister’s underpants (6)
I believe the answer is:
briefs
'barrister's underpants' is the definition.
'briefs' can be an answer for 'underpants' (I've seen this before). I'm unsure of the 'barrister's' bit.
'barrister's underpants' is the wordplay.
'barrister' becomes 'brief' (I've seen this in other clues).
'underpants' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'brief'+'s'='BRIEFS'
(Other definitions for briefs that I've seen before include "Garments" , "Case summaries drawn up for barristers" , "Underpants for lawyers?" , "Underpants for a lawyer" , "Digests" .)