Roll up on opening of Goodwood to get the measure of horse races (7)
I believe the answer is:
furlong
'the measure of horse races' is the definition.
(unit used when measuring horse races)
'roll up on opening of goodwood' is the wordplay.
'roll up' becomes 'furl' (as opposed to unfurl).
'opening of' suggests taking the first letters.
The initial letter of 'goodwood' is 'g'.
'furl'+'on'+'g'='FURLONG'
'to get' acts as a link.
(Other definitions for furlong that I've seen before include "Measure of distance" , "Imperial unit of length, an eighth of a mile" , "220 yards" , "One eighth of a mile" , "Measure of length" .)