As sledge attachments, they move quickly on the track (7)
I believe the answer is:
runners
'as sledge attachments' is the definition.
I can't tell whether this definition defines the answer.
'they move quickly on the track' is the wordplay.
I cannot really understand how this works, but
'move' could be 'run' (run is a kind of move) and 'run' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for runners that I've seen before include "They deliver messages on foot" , "messengers?" , "Joggers" , "Horizontal branches that produce new plants" , "Branches from the base of plant that produce new plants" .)