Nothing going on? (8,4)

I believe the answer is:

birthday suit

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'nothing' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for birthday suit that I've seen before include "Invisible threads?" , "Invisible attire?" , "Nothing to wear?" , "State of total nakedness" , "Invisible outfit?" .)

I've seen this clue in The Times.

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