Swear love in a Volkswagen (4)
I believe the answer is:
avow
'swear' is the definition.
('avow' can be a synonym of 'swear')
'love in a volkswagen' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'in' indicates putting letters inside.
'volkswagen' becomes 'vw' (abbreviation for the car manufacturer).
'a'+'vw'='avw'
'o' inserted inside 'avw' is 'AVOW'.
(Other definitions for avow that I've seen before include "Strongly maintain" , "Asseverate" , "State openly" , "Affirm, swear" , "Solemnly declare" .)