Ensure (9)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
guarantee
'ensure' is the definition.
(I know that ensure can be written as guarantee)
This is all the clue.
(Other definitions for guarantee that I've seen before include "Promising" , "Undertake" , "Written assurance, pledge" , "Something ensuring a particular outcome" , "security system" .)