Empty vehicle one gets into (4)
I believe the answer is:
vain
'empty' is the definition.
(thesaurus)
'vehicle one gets into' is the wordplay.
'vehicle' becomes 'van' (van is a kind of vehicle).
'one' becomes 'i' (Roman numeral).
'gets into' indicates putting letters inside.
'van' enclosing 'i' is 'VAIN'.
(Other definitions for vain that I've seen before include "Conceited, proud of appearance" , "Forever looking in the mirror" , "Conceited - or a waste of time" , "Conceited - futile" , "Concieted or pointless" .)