Make heavy demands on island transport for hire (4)
I believe the answer is:
taxi
'transport for hire' is the definition.
(I've seen this before)
'make heavy demands on island' is the wordplay.
'make heavy demands on' becomes 'tax' (I've seen this before).
'island' becomes 'i'.
'tax'+'i'='TAXI'
(Other definitions for taxi that I've seen before include "US currency" , "Move along a runway" , "Motor-cab" , "people carrier?" , "(Of a plane) move slowly on the ground" .)