Northerner one's excluded from racecourse (4)
I believe the answer is:
scot
'northerner' is the definition.
(I've seen this before)
'one's excluded from racecourse' is the wordplay.
'one' becomes 'a'.
'excluded from' indicates named letters should be taken away.
'racecourse' becomes 'ascot' (I've seen this before).
'ascot' with 'a' taken out is 'SCOT'.
(Other definitions for scot that I've seen before include "No longer tax" , "Native of country of the UK" , "Northern member of UK" , "---- free: originally free from penalties of taxation" , "one from the North of Britain" .)