Join one unable to drive alone in car (6)
I believe the answer is:
couple
'join' is the definition.
(coupling is a kind of joining)
'one unable to drive alone in car' is the wordplay.
'one unable to drive alone' becomes 'l' (a learner driver with L-plates on their car).
'in' indicates putting letters inside.
'car' becomes 'coupe' (coupe is a kind of car).
'l' going inside 'coupe' is 'COUPLE'.
(Other definitions for couple that I've seen before include "Pair of attached people" , "Link together - say two married people" , "item" , "Two of a kind" , "Pair - put together" .)