Make a mistake in bumper race (3)
I believe the answer is:
Here is my best explanation:
'make a mistake' is the definition.
('err' can be a synonym of 'make a mistake')
'in bumper race' is the subsidiary indication.
'in' indicates the answer is hidden within the clue.
'err' is hidden in the letters of 'bumper race'.
(Other definitions for err that I've seen before include "Miss the mark", "Transgress", "Go wrong", "Slip up", "'to... is human,to forgive divine'", "Blunder", "Miscalculate", "Deviate", "Make mistakes" and "Stray from the straight and narrow".)