Supporter after drink outside (7)
I believe the answer is:
apostle
'supporter' is the definition.
(apostle is a kind of supporter)
'after drink outside' is the wordplay.
'after' becomes 'post' (I've seen this before).
'drink' becomes 'ale' (ale is a kind of drink).
'outside' means one lot of letters goes inside another.
'post' placed inside 'ale' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "Principal champion" , "Augustine, say" , "Vigorous supporter" , "Evangelist, proponent" , "Preacher perhaps" .)