Countermand order I've set in motion (8)
'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).
'orderive' is an anagram of 'OVERRIDE'.
(Other definitions for override that I've seen before include "Outweigh; cancel", "Disregard", "Use one's authority to reject or cancel a decision", "Take manual control", "Set aside by superior authority".)