Severe internal gastric trouble (6)

I believe the answer is:

strict

I'm a little stuck... Click here to teach me more about this clue!

'severe' is the definition.
(I've seen this before)

'internal gastric trouble' is the wordplay.
'STRICT' is hidden within the letters.
But, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for strict that I've seen before include "Demanding that rules are observed" , "Not allowing laxity" , "No-nonsense" , "Unbending, precise" , "Harsh and severe" .)

I've seen this clue in The Globe and Mail and The Guardian.

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