Bar legitimate son going in for promoter (9)

I believe the answer is:

publicist

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'for promoter' is the definition.
(I've seen this before)

'bar legitimate son going in' is the wordplay.
'bar' becomes 'pub' (I've seen this in another clue).
'legitimate' becomes 'licit' (synonyms).
'son going' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'licit' enclosing 's' is 'licist'.
'pub'+'licist'='PUBLICIST'

Can you help me to learn more?

(Other definitions for publicist that I've seen before include "Advertising agent" , "Press agent" , "business promoter" , "PR expert" , "PR person" .)

I've seen this clue in The Daily Express.

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