Lad underneath vehicle shows bottle (6)
I believe the answer is:
carboy
'bottle' is the definition.
(carboy is a kind of bottle)
'lad underneath vehicle' is the wordplay.
'lad' becomes 'boy' (I've seen this before).
'underneath' says to put letters next to each other (I've seen this in other clues).
'vehicle' becomes 'car' (car is a kind of vehicle).
'boy' after 'car' is 'CARBOY'.
'shows' acts as a link.
(Other definitions for carboy that I've seen before include "Large bottle in a protective container - by Cora (anag)" , "Large round bottle in wicker casing" , "Large bottle for corrosive liquid" , "Large bottle for holding corrosive liquids" , "Large glass container" .)