In favour of member warning about introduction (12)

I believe the answer is:

prolegomenon

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'introduction' is the definition.
(thesaurus)

'in favour of member warning about' is the wordplay.
'in favour' becomes 'pro' (I've seen this before).
'of member' becomes 'leg' (I can't explain this - if you can you should believe this answer much more).
'warning' becomes 'omen' (omen can mean a warning or sign of the future).
'about' becomes 'on' (as in a book 'on' a certain topic).
'pro'+'leg'+'omen'+'on'='PROLEGOMENON'

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(Other definitions for prolegomenon that I've seen before include "introduction to work" , "Formal, critical introduction to a lengthy text" .)

I've seen this clue in The Times.

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