Course isn’t commonly free, not the first (7)
I believe the answer is:
aintree
'course' is the definition.
(I've seen this before)
'isn't commonly free not the first' is the wordplay.
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
'not the first' means to remove the first letter.
'free' with its first letter taken away is 'ree'.
'aint'+'ree'='AINTREE'
(Other definitions for aintree that I've seen before include "place for horse racing" , "Home of the Grand National" , "Here they run" , "Venue of the Grand National race" , "Merseyside racecourse" .)