Somewhat undervalued engine fuel (4)
I believe the answer is:
derv
'engine fuel' is the definition.
(I've seen this before)
'somewhat undervalued' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'DERV' can be found hidden inside 'undervalued'.
(Other definitions for derv that I've seen before include "Water monster" , "Type of fuel" , "Vehicle fuel oil" , "Lorry fuel" , "Fuel for lorries" .)