Crossword Genius

Needed a kind of oil (9)

Ross

I believe the answer is:

essential

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'needed' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

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(Other definitions for essential that I've seen before include "organic" , "Vital" , "Vitally important" , "Indispensible" , "something necessary" .)

I've seen this clue in The Everyman puzzle in The Observer.
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