Crossword Genius

Stones number one track changes here (3,4)

Ross

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" .)

I've seen this clue in The Guardian.
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