Something of value revealed by schemer, alderman observed (7)

I believe the answer is:

emerald

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'something of value' is the definition.
I can't judge whether this definition defines the answer.

'revealed by schemer alderman observed' is the wordplay.
'revealed by' indicates the answer is hidden within the clue.
'EMERALD' is hidden within 'schemer alderman observed'.

Can you help me to learn more?

(Other definitions for emerald that I've seen before include "Green like this isle" , "Bright green colour" , "Gem; type of moth" , "and 1 Across: Ireland" , "Green gem" .)

I've seen this clue in The Irish Times.

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