How to make a cancellation fully in order? (7)
I believe the answer is:
nullify
'how to make a cancellation' is the definition.
(to nullify can mean to cancel)
'fully in order?' is the wordplay.
'order?' indicates anagramming the letters (I've seen 'order' mean this).
'fully'+'in'='fullyin'
'fullyin' is an anagram of 'NULLIFY'.
(Other definitions for nullify that I've seen before include "Invalidate" , "Make void" , "Render ineffective" , "Make invalid" , "Declare invalid" .)