Underwrite (6)
I believe the answer is:
insure
'underwrite' is the definition.
(I know that underwrite can be written as insure)
This is the entire clue.
(Other definitions for insure that I've seen before include "Take out a policy" , "Guarantee against loss" , "Guarantee; make secure" , "Arrange for payment in the event of loss" , "Protect by taking out a policy" .)