French article increased in value, but is not recognised (13)

I believe the answer is:

unappreciated

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'french article increased in value but' is the definition.
I can't tell whether this definition defines the answer.

'not recognised' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'recognised' becomes 'appreciated' (appreciating is a kind of recognizing).
'un'+'appreciated'='UNAPPRECIATED'

'is' is the link.

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I've seen this clue in The Daily Express.

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