A northerner, of course (5)
I believe the answer is:
ascot
'course' is the definition.
(Ascot is a racecourse)
'a northerner' is the wordplay.
'northerner' becomes 'scot' (someone from Scotland).
'a'+'scot'='ASCOT'
'of' is the link.
(Other definitions for ascot that I've seen before include "Famous racecourse" , "Fungus affecting wood" , "Royal race course" , "Racecourse with royal associations" , "Posh racecourse" .)