Vehicle with a futon inside (4)
I believe the answer is:
auto
'vehicle' is the definition.
(auto is a kind of vehicle)
'with a futon inside' is the wordplay.
'with' says to put letters next to each other.
'inside' says to take the centre.
The central letters of 'futon' are 'uto'.
'a' next to 'uto' is 'AUTO'.
(Other definitions for auto that I've seen before include "Car (American)" , "Motor car (mainly US)" , "American vehicle" , "Motor vehicle, in short" , "Car (chiefly in US)" .)