When a car refuels, containers fall over (3,4)
I believe the answer is:
pit stop
'when a car refuels' is the definition.
'containers fall over' is the wordplay.
'containers' becomes 'pots' (pot is a kind of container).
'fall' becomes 'tip'.
'over' says the letters should be written backwards.
'pots'+'tip'='potstip'
'potstip' in reverse letter order is 'PIT STOP'.
(Other definitions for pit stop that I've seen before include "When tripping, break" , "F1 race feature" , "Break for refuelling" , "part of the race" , "Break for fuel etc." .)