Gemstone for mere lad (7)

I believe the answer is:

emerald

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

'gemstone' is the definition.
(I know that emerald is a type of gemstone)

'mere lad' is the wordplay.
'mere lad' anagrams to 'EMERALD'.
But, I am unsure how the anagram is indicated.

'for' is the link.

Can you help me to learn more?

(Other definitions for emerald that I've seen before include "and 1 Across: Ireland" , "Highly-valued variety of beryl" , "Green like this isle" , "Mere lad (anag.)" , "Bright green colour" .)

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

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