Make heavy demands on one vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'make heavy demands on one' is the wordplay.
'make heavy demands on' becomes 'tax' (I've seen this before).
'one' becomes 'i' (Roman numeral**).
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "Minicab" , "Part of fleet" , "type of vehicle" , "Public service vehicle" , "Motor cab" .)