No laughing matter for hasty pen (3)

I believe the answer is:

sty

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'pen' is the definition.
(both can mean an animal enclosure)

'no laughing matter for hasty' is the wordplay.
'STY' is hidden within the letters.
But, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for sty that I've seen before include "Farmyard enclosure" , "Pigs' pen" , "Pig house" , "Filthy place" , "Pig-pen and infection on the eyelid" .)

I've seen this clue in the Sydney Morning Herald.

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