Course record therefore momentous, originally (5)
I believe the answer is:
epsom
'course' is the definition.
(a race course)
'record therefore momentous originally' is the wordplay.
'record' becomes 'EP' (abbreviation for extended play record).
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'originally' indicates taking the first letters.
The first letter of 'momentous' is 'm'.
'ep'+'so'+'m'='EPSOM'
(Other definitions for epsom that I've seen before include "English town, of course?" , "Mares would triumph here" , "Derby area" , "track in Surrey" , "Classic racecourse" .)