Crossword Genius

What an engine may consume in overdrive, possibly? (4)

Ross

I believe the answer is:

derv

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

'overdrive possibly?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.
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