Reverse someone else's decision (8)
I believe the answer is:
overrule
'reverse someone else's decision' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for overrule that I've seen before include "Impose a decision on" , "Disallow by using one's superior authority" , "Reverse another's decision" , "Don't allow" , "Set aside decision by superior authority" .)