Entrance contains new sponsor (5)
I believe the answer is:
donor
'sponsor' is the definition.
(I've seen this before)
'entrance contains new' is the wordplay.
'entrance' becomes 'door' (door is a kind of entrance).
'contains' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'door' going around 'n' is 'DONOR'.
(Other definitions for donor that I've seen before include "A giver, often of blood" , "Someone providing" , "Generous type" , "Maker of gift" , "blessed person, relatively" .)