Make heavy demands on independent transport (4)
I believe the answer is:
taxi
'transport' is the definition.
(taxi is a kind of transport)
'make heavy demands on independent' is the wordplay.
'make heavy demands on' becomes 'tax' (I've seen this before).
'independent' becomes 'i'.
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "Motor vehicle for hire with driver" , "(Plane) roll over tarmac" , "Hired vehicle" , "Prepare to take off" , "At airport, guide" .)