'Heroin kept inside' heard from pen inmate? (4)

I believe the answer is:

oink

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'heard from pen inmate?' is the definition.
I can't judge whether this definition defines the answer.

'heroin kept inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'OINK' is hidden within 'heroin kept'.

Can you help me to learn more?

(Other definitions for oink that I've seen before include "farmyard noise" , "what we hear from Berkshire?" , "Beastly noise" , "Piggy noise" , "Animal sound" .)

I've seen this clue in The Guardian.

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