Support for transport workers led by one in energy unit (7)
I believe the answer is:
bitumen
'support for transport' is the definition.
(mixture on road surfaces)
'workers led by one in energy unit' is the wordplay.
'workers' becomes 'men' (men can mean staff or employees).
'led by' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'in' indicates putting letters inside.
'energy unit' becomes 'btu' (btu is a kind of energy unit).
'i' put into 'btu' is 'bitu'.
'men' put after 'bitu' is 'BITUMEN'.
(Other definitions for bitumen that I've seen before include "Substance for surfacing roads" , "Material for road surfacing, roofing, etc" , "Tarry substance used to surface roads and roofs" , "Road's topping" , "Road-surfacing material" .)