Car was effective for one not needing instructor (10)
I believe the answer is:
autodidact
'for one not needing instructor' is the definition.
(I've seen this before)
'car was effective' is the wordplay.
'car' becomes 'auto' ('auto' can be a synonym of 'car').
'was effective' becomes 'didact'.
'auto'+'didact'='AUTODIDACT'
(Other definitions for autodidact that I've seen before include "Self-taught person" , "Someone self-taught" , "learner without instructor" , "I skipped school" , "person who taught himself" .)