Fuel somewhat undervalued (4)
I believe the answer is:
derv
'fuel' is the definition.
(I know that derv is a type of diesel fuel)
'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 "Type of fuel" , "Fuel for lorries" , "Lorry fuel" , "Water monster" , "Vehicle fuel oil" .)