Cold stew in tremendous vat (15)

I believe the answer is:

undemonstrative

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

'stew in tremendous vat' is the wordplay.
'stew' indicates anagramming the letters.
'in'+'tremendous'+'vat'='intremendousvat'
'intremendousvat' is an anagram of 'UNDEMONSTRATIVE'.

(Another definition for undemonstrative that I've seen is " Cold".)

I've seen this clue in The Telegraph.

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