In essence, the banks' national treasure (6)

I believe the answer is:

nature

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'in essence the banks'' is the definition.
I can't judge whether this defines the answer.

'national treasure' is the wordplay.
'national' becomes 'nat'.
'treasure' becomes 'ure' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'nat'+'ure'='NATURE'

Can you help me to learn more?

(Other definitions for nature that I've seen before include "Character (of a person or thing)" , "Inborn character; essence" , "Picturesque countryside" , "Mother Earth" , "Inherent character" .)

I've seen this clue in the Irish Times.

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