Opportunity to welcome new supporter (5)
I believe the answer is:
donor
'supporter' is the definition.
(I've seen this before)
'opportunity to welcome new' is the wordplay.
'opportunity' becomes 'door' (when one door closes another opens).
'to welcome' indicates putting letters inside (inserted letters are welcomed in).
'new' becomes 'n' (abbreviation as in New Testament NT).
'door' going around 'n' is 'DONOR'.
(Other definitions for donor that I've seen before include "One giving to a cause" , "One who gives, maybe blood" , "Someone providing" , "One giving something" , "One who gives, perhaps blood" .)