Divert to subsidiary racecourse (9)
I believe the answer is:
sidetrack
'divert' is the definition.
(I know that sidetrack is a more specific form of the action divert)
'subsidiary racecourse' is the wordplay.
'subsidiary' becomes 'side' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'racecourse' becomes 'track' (synonyms).
'side'+'track'='SIDETRACK'
'to' is the link.
Can you help me to learn more?
(Other definitions for sidetrack that I've seen before include "Distract (from big issue)" , "Provide diversion" , "Bit of railway" , "lead away" , "small way" .)