Some undervalued fuel (4)
I believe the answer is:
derv
'fuel' is the definition.
(I know that derv is a type of diesel fuel)
'some undervalued' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'DERV' can be found hidden inside 'undervalued'.
(Other definitions for derv that I've seen before include "Vehicle diesel-oil" , "Type of fuel" , "Diesel engine fuel oil" , "Vehicle fuel oil" , "Fuel for lorries" .)