Hemmed in by two vehicles is a large vehicle (7)
I believe the answer is:
caravan
'large vehicle' is the definition.
'caravan' can be an answer for 'vehicle' (caravan is a kind of vehicle). I'm unsure of the 'large' bit.
'hemmed in by two vehicles' is the wordplay.
'two vehicles' means two replacements for 'vehicle'.
'hemmed in' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'vehicle' becomes 'car' (car is a kind of vehicle).
'vehicle' becomes 'van' (type of vehicle).
'a' put after 'car' is 'cara'.
'cara'+'van'='CARAVAN'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for caravan that I've seen before include "Mobile home" , "Motor-home" , "Holiday vehicle" , "Desert convoy" , "Home on wheels" .)