Advocates acting on brief instructions (10)
I believe the answer is:
barristers
'instructions' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.
'advocates acting on brief' is the wordplay.
'advocates acting' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'on' says to put letters next to each other.
'brief' becomes 'barrister' (brief can mean a lawyer).
's' put after 'barrister' 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" .)