No longer a concert piece (4)

I believe the answer is:

once

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'no' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

'longer a concert piece' is the wordplay.
'piece' indicates a hidden word.
'ONCE' is hidden within 'longer concert'.

Can you help me to learn more?

(Other definitions for once that I've seen before include "At some former time" , "In the old days" , "Formerly; as soon as" , "not repeatedly" , "Previously" .)

I've seen this clue in the Evening Standard.

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