Stolid fiends stuck in rocky ravine tut (15)

I believe the answer is:

undemonstrative

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

'stolid' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'fiends stuck in rocky ravine tut' is the wordplay.
'fiends' becomes 'demons' ('demon' can be a synonym of 'fiend').
'stuck in' is an insertion indicator.
'rocky' indicates an anagram (letters rocked or thrown around).
'ravine'+'tut'='ravinetut'
'ravinetut' with letters rearranged gives 'untrative'.
'demons' inserted within 'untrative' is 'UNDEMONSTRATIVE'.

Can you help me to learn more?

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

I've seen this clue in The Daily Express.

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