Regretful having traded small for large vehicle (5)
I believe the answer is:
lorry
'vehicle' is the definition.
(lorry is a kind of vehicle)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for lorry that I've seen before include "Heavy goods vehicle" , "Load-carrying vehicle" , "Commercial vehicle" , "Large motor vehicle for transporting goods" , "Industrial vehicle" .)
