Reticent devils in random ravine tut (15)

I believe the answer is:

undemonstrative

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'reticent' is the definition.
(similar in meaning)

'devils in random ravine tut' is the wordplay.
I cannot quite see how this works, but
'devils' could be 'demons' ('demon' can be a synonym of 'devil') and 'demons' is present in the answer.
This explanation may well be incorrect...

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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