Not doing much business providing fuel (5)

I believe the answer is:

slack

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'fuel' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for slack that I've seen before include "Loose" , "Limp - negligent" , "Lax" , "Flaccid" , "skive" .)

I've seen this clue in The Irish News.

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