Episcopal stone? (4)

I believe the answer is:

opal

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

'stone?' is the definition.
(gemstone)

'episcopal' is the wordplay.
'OPAL' is hidden within the letters.
But, I'm uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Milky stuff that's hard" , "Milky iridescent stone" , "Translucent gemstone" , "Semi-transparent gemstone" , "it may be highly valued" .)

I've seen this clue in the Evening Standard.

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