Pledge — guarantee (6)
I believe the answer is:
surety
This clue is a double definition.
'pledge' is the first definition.
(I know that surety is a type of guarantee)
'guarantee' is the second definition.
(I know that surety is a type of guarantee)
(Other definitions for surety that I've seen before include "Money given as guarantee of action" , "Legal safeguard against loss" , "Semitic language" , "Safeguard given against financial loss" , "Guarantee (for pledge)" .)