Running tracks! (5)
I believe the answer is:
rails
This could be a double definition.
'running' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'tracks' is the second definition.
('rail' can be a synonym of 'track')
Can you help me to learn more?
(Other definitions for rails that I've seen before include "Water birds" , "Train tracks" , "Wading birds - dysfunctional person has come off them" , "For trains" , "secretive birds" .)