He enters commercial lease for supporter (8)
I believe the answer is:
adherent
'for supporter' is the definition.
(I've seen this in another clue)
'he enters commercial lease' is the wordplay.
'enters' means one lot of letters goes inside another.
'commercial' becomes 'ad' (abbreviation for advertisement).
'lease' becomes 'rent' (synonyms).
'ad'+'rent'='adrent'
'he' put within 'adrent' is 'ADHERENT'.
(Other definitions for adherent that I've seen before include "Follower, partisan" , "Loyal follower - sticks close" , "Person supporting" , "Sticking; partisan" , "Freak" .)