Vehicle not going back into case (6)
I believe the answer is:
carton
'case' is the definition.
(thesaurus)
'vehicle not going back' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle**).
'going back' shows that the letters should be reversed in order.
'not' back-to-front is 'ton'.
'car'+'ton'='CARTON'
'into' is the link.
(Other definitions for carton that I've seen before include "fictional lawyer" , "Cardboard or plastic container" , "Box made of cardboard" , "Dickensian hero" , "Flapped cardboard box" .)