Fuel that is partly undervalued (4)
I believe the answer is:
derv
'fuel' is the definition.
(derv is a kind of fuel)
'partly undervalued' is the wordplay.
'partly' indicates a hidden word.
'DERV' is hidden within 'undervalued'.
'that is' is the link.
(Other definitions for derv that I've seen before include "Fuel for lorries" , "Lorry fuel" , "Vehicle fuel oil" , "Diesel engine fuel oil" , "Water monster" .)