Regularly curved track? (8)
I believe the answer is:
crescent
'regularly curved track?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'regularly curved track?' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'track?' becomes 'scent' (both can mean a trace left by someone).
The alternating letters of 'curved' are 'cre'.
'cre'+'scent'='CRESCENT'
(Other definitions for crescent that I've seen before include "Shape of waxing or waning moon" , "Waxing process could show one" , "Way" , "Curved street or terrace" , "Turkish emblem" .)