A rock band (7,4)

I believe the answer is:

diamond ring

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'a rock band' is the definition.
The definition and answer are not the same part of speech.

'rock band' is the wordplay.
'rock' becomes 'diamond'.
'band' becomes 'ring' (a ring is a band of metal).
'diamond'+'ring'='DIAMOND RING'

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I've seen this clue in the Evening Standard.

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