Somewhat undervalued fuel (4)
I believe the answer is:
derv
'fuel?' is the definition.
(derv is a kind of fuel)
'somewhat undervalued' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'DERV' is hidden in the letters of 'undervalued'.
(Other definitions for derv that I've seen before include "Diesel engine fuel oil" , "Water monster" , "Lorry fuel" , "Vehicle diesel-oil" , "Fuel for lorries" .)