Stone confused mere lad (7)

I believe the answer is:

emerald

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'stone' is the definition.
(type of gemstone)

'confused mere lad' is the wordplay.
'mere lad' can be anagrammed to 'EMERALD'.
However, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for emerald that I've seen before include "Small size of type, in printing" , "Highly-valued variety of beryl" , "and 13 Across: Poetic name for Ireland" , "On of the Greens" , "Shade of green" .)

I've seen this clue in The Mirror.

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