Convincing oneself car has motion (14)
I believe the answer is:
autosuggestion
'convincing oneself' is the definition.
(psychological acceptance of an idea from oneself)
'car has motion' is the wordplay.
'car' becomes 'auto' (synonyms).
'has' says to put letters next to each other.
'motion' becomes 'suggestion' (a motion is a suggestion at a meeting).
'auto'+'suggestion'='AUTOSUGGESTION'
(Other definitions for autosuggestion that I've seen before include "hypnotism?" , "How we can persuade ourselves" .)