Gastric tension holds firm (6)

I believe the answer is:

strict

'firm' is the definition.
(firm can mean strict or severe)

'gastric tension holds' is the wordplay.
'holds' says the answer is hidden in the clue.
'STRICT' is hidden in the letters of 'gastric tension'.

(Other definitions for strict that I've seen before include "Exact, rigorous" , "Harsh and severe" , "Requiring exact obedience or compliance with rules" , "No-nonsense" , "Rigorous, uncompromising" .)

I've seen this clue in The Daily Express.

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