Advocate stone being used in wall? (9)
I believe the answer is:
barrister
'advocate' is the definition.
(thesaurus)
'stone being used in wall?' is the wordplay.
'stone' becomes 'st'.
'being used in' is an insertion indicator.
'wall?' becomes 'barrier' (I've seen this before).
'st' placed inside 'barrier' is 'BARRISTER'.
(Other definitions for barrister that I've seen before include "One represents clients in court" , "Legal counsel" , "Legal representative" , "Lawyer for the higher courts" , "Legal advocate, must be briefed" .)