Will post on behalf of? That's champion! (5,2,3)

I believe the answer is:

stick up for

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'champion' is the definition.
(thesaurus)

'will post on behalf of?' is the wordplay.
'will post' becomes 'stickup' (I am not sure about this - if you are sure you should believe this answer much more).
'on behalf of?' becomes 'for' (I've seen this in another clue).
'stickup'+'for'='STICK UP FOR'

'that's' is the link.

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I've seen this clue in the Evening Standard.

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