Such fuel could appear a bit undervalued (6)
I believe the answer is:
diesel
'such fuel' is the definition.
'diesel' can be an answer for 'fuel' (I've seen this before). I'm not sure about the 'such' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for diesel that I've seen before include "See lid, oddly, of the fuel" , "See lid for the engine" , "Rudolf __ , French-born German engine inventor, d.1913" , "Kind of engine" , "Lorry fuel" .)