Barrister put on small underwear (6)
I believe the answer is:
briefs
'underwear' is the definition.
(I know that briefs is a type of lingerie)
'barrister put on small' is the wordplay.
'barrister' becomes 'brief' (I've seen this before).
'put on' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'brief'+'s'='BRIEFS'
(Other definitions for briefs that I've seen before include "Orders" , "Summaries of legal cases" , "They are prepared for barristers' perusal" , "Underpants for a lawyer" , "Documents stating facts and points of law" .)