Worst and best part of a motor race? (3,4)
I believe the answer is:
pit stop
'part of a motor race?' is the definition.
(I know that pit stop is part of auto race)
'worst and best' is the wordplay.
'worst' becomes 'pits' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'best' becomes 'top' (top can mean the very best).
'pits'+'top'='PIT STOP'
(Other definitions for pit stop that I've seen before include "F1 race feature" , "period of service?" , "for rest of travellers" , "part of the race" , "When tripping, break" .)