Great golfer periodically upset scorer (5)
I believe the answer is:
elgar
'scorer' is the definition.
(Edward Elgar)
'great golfer periodically upset' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'upset' shows that the letters should be reversed in order.
'great'+'golfer'='greatgolfer'
The alternating letters of 'greatgolfer' are 'ragle'.
'ragle' in reverse letter order is 'ELGAR'.
(Other definitions for elgar that I've seen before include "13 composer, 1857-1934" , "Glare critically at the British composer" , "[NIMROD] 's creator" , "Glare at great English composer" , "Land of Hope and Glory composer" .)