Driving test obstacle (4)
I believe the answer is:
cone
'driving test obstacle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cone that I've seen before include "Hollow object associated with ice cream and motorways" , "16 Cornet" , "standing in middle of motorway" , "Volcanic peak" , "Dry fruit of many evergreen trees, say pine" .)