They run on tracks (6)
I believe the answer is:
trains
'they run on tracks' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trains that I've seen before include "Pieces of cloth trailing from the backs of gowns" , "Exercises" , "Aims; coaches" , "They're drawn by locomotive" , "Prepares for competition" .)