Briefs: they hinder union, Stone admitted (10)
I believe the answer is:
barristers
'briefs they' is the definition.
The definition suggests a plural noun which matches the answer.
'hinder union stone admitted' is the wordplay.
'hinder union' becomes 'barriers' (I can't explain this - if you can you should give a lot more credence to this answer).
'stone' becomes 'st' (abbreviation).
'admitted' indicates putting letters inside (inserted letters are admitted in).
'barriers' placed around 'st' is 'BARRISTERS'.
Can you help me to learn more?
(Other definitions for barristers that I've seen before include "they stand up in court?" , "Lawyers" .)