Cancel a golfer's last round (7)
I believe the answer is:
annular
'round' is the definition.
(thesaurus)
'cancel a golfer's last' is the wordplay.
'cancel' becomes 'annul' (annulling is a kind of cancelling**).
'last' suggests the final letters.
The last letter of 'golfer' is 'r'.
'annul'+'a'+'r'='ANNULAR'
(Other definitions for annular that I've seen before include "in a band" , "Circular" , "Ringlike; type of eclipse" , "Doughnut-shaped -- a LAN run (anag)" , "Ring-shaped" .)