Make heavy demands on current vehicle (4)
I believe the answer is:
taxi
'vehicle' is the definition.
(taxi is a kind of vehicle)
'make heavy demands on current' is the wordplay.
'make heavy demands on' becomes 'tax' (I've seen this before).
'current' becomes 'i' (symbol for electric current).
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "I hail you!" , "run along the ground" , "Minicab" , "(Of a plane) move slowly on the ground" , "Rank occupant" .)