Cancel a Republican circular (7)
I believe the answer is:
annular
'circular' is the definition.
(synonyms)
'cancel a republican' is the wordplay.
'cancel' becomes 'annul' (annulling is a kind of cancelling).
'republican' becomes 'r' (abbreviation).
'annul'+'a'+'r'='ANNULAR'
(Other definitions for annular that I've seen before include "in a band" , "Ring-shaped" , "Forming a ring" , "Doughnut-shaped -- a LAN run (anag)" , "Ringlike; type of eclipse" .)