Settle up in advance (6)
I believe the answer is:
prepay
'settle up in advance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for prepay that I've seen before include "Offer retainer" , "stump up ahead" , "Give money (for goods) in advance" , "Remit for something before receiving it" , "Settle in advance" .)