Own a car with nothing inside (4)
I believe the answer is:
avow
'own' is the definition.
(I've seen this in another clue)
'a car with nothing inside' is the wordplay.
'car' becomes 'VW' (Volkswagen car).
'with' is a charade indicator (letters next to each other).
'nothing' becomes 'o' (O resembles 0).
'inside' is an insertion indicator.
'a'+'vw'='avw'
'avw' enclosing 'o' is 'AVOW'.
(Other definitions for avow that I've seen before include "Declare - admit" , "Assert or confess openly" , "Assert/confess openly" , "Assert - admit" , "Declare a promise of fidelity at the altar" .)