Nearly all use a self-starter during a car test (4)
I believe the answer is:
most
'nearly all' is the definition.
'self-starter during a car test' is the wordplay.
'starter' indicates taking the first letters.
'during' indicates putting letters inside.
'a car test' becomes 'MOT'.
The initial letter of 'self' is 's'.
's' inserted inside 'mot' is 'MOST'.
'use a' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for most that I've seen before include "In the highest degree" , "Extremely, very" , "Nearly all, majority" , "Kind of commercial pact where two countries agree to treat each other as well as any other" , "Almost all" .)