Support one national organisation's spokesperson (9)

I believe the answer is:

proponent

'spokesperson' is the definition.
(thesaurus)

'support one national organisation's' is the wordplay.
'support' becomes 'prop' (prop is a kind of support).
'national organisation' becomes 'nt' (I've seen this in other clues).
'prop'+'one'+'nt'='PROPONENT'

(Another definition for proponent that I've seen is " Advocate (of a theory, etc)".)

I've seen this clue in The Telegraph.

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