Cancel, on principle! (8)
I believe the answer is:
overrule
'cancel' is the definition.
(thesaurus)
'on principle' is the wordplay.
'on' becomes 'over' (over can mean on top of).
'principle' becomes 'rule' (synonyms).
'over'+'rule'='OVERRULE'
(Other definitions for overrule that I've seen before include "Disallow, set aside" , "Impose a decision on" , "Disallow by using one's superior authority" , "Reverse someone else's decision" , "Turn down (objection)" .)