Opening payment (4)
I believe the answer is:
rent
I believe this clue is a double definition.
'opening' is the first definition.
(I know that rent is a type of opening)
'payment' is the second definition.
(I know that rent is a type of annuity in advance)
(Other definitions for rent that I've seen before include "Split - lease" , "Lease - torn apart" , "Payment due to landlord" , "Periodic payment as for accommodation" , "It's paid by a tenant" .)