Undervalued fuel? (4)

I believe the answer is:

derv

I'm a little stuck... Click here to teach me more about this clue!

'fuel?' is the definition.
(derv is a kind of fuel)

'undervalued' is the wordplay.
'DERV' is hidden in amongst the letters.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for derv that I've seen before include "Type of fuel" , "Vehicle diesel-oil" , "Diesel engine fuel oil" , "Fuel for lorries" , "Lorry fuel" .)

I've seen this clue in the Evening Standard.

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