Friend in motoring competition failing to start (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'motoring competition failing to start' is the wordplay.
'motoring competition' becomes 'rally' (rally can mean a motor race).
'failing to start' means to remove the first letter.
'rally' with its first letter removed is 'ALLY'.
'in' is the link.
(Other definitions for ally that I've seen before include "Associate that provides assistance" , "Collaborator, affiliate" , "couple?" , "Co-operating person or nation" , "Someone with whom one is in league" .)