Entrance has new supporter (5)
I believe the answer is:
donor
'supporter' is the definition.
(I've seen this before)
'entrance has new' is the wordplay.
'entrance' becomes 'door' (door is a kind of entrance).
'has' is an insertion indicator.
'new' becomes 'n' (abbreviation as in New Testament NT).
'door' placed around 'n' is 'DONOR'.
(Other definitions for donor that I've seen before include "patron" , "A giver, perhaps of blood" , "blessed person, relatively" , "A giver, often of blood" , "Organ giver?" .)