Spirit unusually useful in the engine (5)

I believe the answer is:

genie

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'spirit' is the definition.
(genie is a kind of spirit)

I cannot understand how the rest of the clue works.

(Other definitions for genie that I've seen before include "Arabian sprite" , "Bottled spirit?" , "Goblin as found in Aladdin's lamp" , "Arabian goblin with strange powers" , "Magic spirit" .)

I've seen this clue in the Evening Standard.

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