Little James sounds a treasure! (3)

I believe the answer is:

jem

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'a treasure' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'little james sounds' is the wordplay.
'little' indicates taking the first letters.
'sounds' becomes 'em' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letter of 'james' is 'j'.
'j'+'em'='JEM'

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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