One of many across the tracks (7)
I believe the answer is:
sleeper
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sleeper that I've seen before include "Kipper" , "Rail supporter; secret agent" , "One having D [DREAM]" , "Express tears over one" , "Long-inactive secret agent" .)