Silk knickers with the rear cut out? (5)
I believe the answer is:
'cut out?' is the definition.
'brief' can be an answer for 'cut' (thesaurus). I'm unsure of the 'out?' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for brief that I've seen before include "Summary of facts for barrister to consider" , "Provide someone with the facts of the case" , "Concise; short-lived" , "Short - work for a barrister" , "Case given to a barrister" .)