Vehicle with black on - soot? (6)
I believe the answer is:
carbon
'soot?' is the definition.
(soot is a kind of carbon)
'vehicle with black on' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'with' means one lot of letters go next to another.
'black' becomes 'b' (as in pencil classifications).
'car'+'b'+'on'='CARBON'
(Other definitions for carbon that I've seen before include "Substance" , "Element of glass" , "Elementary" , "Non-metallic element, C" , "US five-cent coin" .)