Swear there’s nothing in car (3)
I believe the answer is:
vow
'swear' is the definition.
(I've seen this in another clue)
'nothing in car' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'in' indicates putting letters inside.
'car' becomes 'vw' (Volkswagen car).
'o' going within 'vw' is 'VOW'.
'there's' is the link.
(Other definitions for vow that I've seen before include "Promise, swear" , "Solemn pledge" , "Plight" , "Binding promise" , "Oath or solemn promise" .)