Naturally belonging to racetrack (2,6)
I believe the answer is:
of course
'naturally' is the definition.
(I know that naturally can be written as of course)
'belonging to racetrack' is the wordplay.
'belonging to' becomes 'of' ('of' can mean 'belonging to' or 'owned by').
'racetrack' becomes 'course' (I am not sure about this - if you are sure you should believe this answer much more).
'of'+'course'='OF COURSE'
Can you help me to learn more?
(Other definitions for of course that I've seen before include "Certainly, without a doubt" , "'Naturally, as might be expected (2,6)'" , "Without a doubt, as was to be expected" , "without question" , "About a series of lessons" .)