Champion with poster about to appear in election (8)
I believe the answer is:
advocate
'champion' is the definition.
(advocate can mean to champion or support)
'poster about to appear in election' is the wordplay.
'poster' becomes 'ad' (short for advert).
'about' becomes 'ca' (circa).
'to appear in' means one lot of letters goes inside another.
'election' becomes 'vote' (vote can mean an election or poll).
'ca' placed inside 'vote' is 'vocate'.
'ad'+'vocate'='ADVOCATE'
'with' is the link.
(Other definitions for advocate that I've seen before include "Proponent" , "Defender" , "Speak in favour of" , "Brief" , "Argue for" .)