Stones number one track changes here (3,4)
I believe the answer is:
pit stop
'track changes here' is the definition.
(where parts are changed on a racetrack)
'stones number one' is the wordplay.
'stones' becomes 'pits' ('pit' can be a synonym of 'stone').
'number one' becomes 'top' (number one can mean the best).
'pits'+'top'='PIT STOP'
(Other definitions for pit stop that I've seen before include "place where car can be serviced - with 31a" , "GP changing time" , "part of the race" , "brief service" , "Break during motor race" .)