Reveal a policy covering up review by one of The Stones (4)

I believe the answer is:

opal

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'review by one of the stones' is the definition.
I can't judge whether this definition defines the answer.

'reveal a policy covering up' is the wordplay.
'covering' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'lapo' is hidden within 'reveal a policy'.
'lapo' back-to-front is 'OPAL'.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Lustrous stone" , "Glassy gemstone" , "Semi-precious stone of b;uish white colour" , "Form of hydrated silica, often used as a gemstone" , "Variegated gemstone" .)

I've seen this clue in the Irish Times.

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