Much valuable material in the theatre a sure thing (8)

I believe the answer is:

treasure

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

'much valuable material' is the definition.
'treasure' can be an answer for 'material' (thesaurus). I'm not sure about the 'much valuable' bit.

'in the theatre a sure thing' is the wordplay.
'in' indicates a hidden word.
'TREASURE' is hidden within 'theatre a sure thing'.

Can you help me to learn more?

(Other definitions for treasure that I've seen before include "Classic adventure story" , "A collection of precious things" , "Hold dearly, like buried gold etc" , "Highly valued possession" , "Set great store by" .)

I've seen this clue in the Sydney Morning Herald.

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