Laughing matter oddly in Gdansk (3)

I believe the answer is:

gas

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'laughing matter' is the definition.
'gas' can be an answer for 'matter' (gas is a kind of matter). I'm not certain of the 'laughing' bit.

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

Can you help me to learn more?

(Other definitions for gas that I've seen before include "Fumes, vapour" , "Hydrogen, for example" , "Hydrogen possibly" , "What's needed to drive" , "Oxygen maybe" .)

I've seen this clue in The Irish Times.

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