Fuel tanker (6)
I believe the answer is:
bowser
'fuel tanker' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bowser that I've seen before include "Aviation-fuel tanker" , "Fuel or water tanker" , "vehicle for oil" , "Fuelling tanker" , "Tanker for refuelling aircraft" .)