Ambassador seen in commercial with payment from supporter (8)
I believe the answer is:
adherent
'supporter' is the definition.
(I've seen this before)
'ambassador seen in commercial with payment' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'seen in' is an insertion indicator.
'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' put into 'adrent' is 'ADHERENT'.
'from' is the link.
(Other definitions for adherent that I've seen before include "One believes in and helps spread the doctrine of another" , "Loyal follower, one who sticks" , "Supporter, follower" , "Supporter; sticking" , "Devotee" .)