To cancel, one rings (6)
I believe the answer is:
annuli
'rings' is the definition.
('annulus' can be a synonym of 'ring')
'to cancel one' is the wordplay.
'to cancel' becomes 'annul' (annulling is a kind of cancelling).
'one' becomes 'i' (Roman numeral).
'annul'+'i'='ANNULI'
(Other definitions for annuli that I've seen before include "round these parts" , "Areas between two concentric circles" , "Ring-shaped objects" .)