Curved line made by a track periodically (3)
I believe the answer is:
arc
'curved line' is the definition.
(an arc is a part of a curved line)
'by a track periodically' is the wordplay.
'by' says to put letters next to each other.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternating letters of 'track' are 'rc'.
'a' put next to 'rc' is 'ARC'.
'made' is the link.
(Other definitions for arc that I've seen before include "Curved part of circle" , "A curve for Joan of . . ." , "Section of circumference" , "Light" , "Curved path" .)