Distance between the rails of a railway track (5)
I believe the answer is:
gauge
'distance between the rails of a railway track' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gauge that I've seen before include "Distance between the rails of a railway track" , "standard size" , "Measure(r)" , "Adjudge" , "Rail track width" .)