Ambassador in commercial with payment for supporter (8)
I believe the answer is:
adherent
'for supporter' is the definition.
(I've seen this before)
'ambassador in commercial with payment' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'in' means one lot of letters goes inside another.
'commercial' becomes 'ad' (abbreviation for advertisement).
'with' says to put letters next to each other.
'payment' becomes 'rent' (rent is a kind of payment).
'ad'+'rent'='adrent'
'he' placed within 'adrent' is 'ADHERENT'.
(Other definitions for adherent that I've seen before include "Loyal follower, one who sticks" , "Staunch follower" , "Devotee" , "Follower, partisan" , "Dedicated supporter" .)