Countermand order I've set in motion (8)
I believe the answer is:
override
'countermand' is the definition.
(I've seen this before)
'order i've set in motion' is the wordplay.
'set in motion' indicates anagramming the letters (letters made to move around).
'order'+'ive'='orderive'
'orderive' is an anagram of 'OVERRIDE'.
(Other definitions for override that I've seen before include "Set aside by superior authority" , "Use one's authority to reject or cancel a decision" , "Counteract the normal operation" , "Supersede" , "Countermand" .)