It's very enjoyable, being in funds (3)

I believe the answer is:

fun

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'funds' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

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

Can you help me to learn more?

(Other definitions for fun that I've seen before include "'Merriment, playfulness (3)'" , "Entertaining" , "entertainment" , "Playful amusement" , "Light-hearted pleasure" .)

I've seen this clue in the Evening Standard.

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