Stone used in wall, say, in brief (9)

I believe the answer is:

barrister

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'in brief' is the definition.
'barrister' can be an answer for 'brief' (I've seen this before). I'm unsure of the 'in' bit.

'stone used in wall' is the wordplay.
'stone' becomes 'st'.
'used in' indicates putting letters inside.
'wall' becomes 'barrier' (I've seen this before).
'st' going into 'barrier' is 'BARRISTER'.

'say' is the link.

Can you help me to learn more?

(Other definitions for barrister that I've seen before include "Legal representative" , "Legal advocate, must be briefed" , "Court lawyer" , "American lawyer" , "member of an inn" .)

I've seen this clue in The Telegraph.

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