Music producers unfortunately pain this group (7,6)

I believe the answer is:

upright pianos

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'music producers' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how they can define each other.

'unfortunately pain this group' is the wordplay.
'unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'pain'+'this'+'group'='painthisgroup'
'painthisgroup' anagrammed gives 'UPRIGHT PIANOS'.

Can you help me to learn more?

(Another definition for upright pianos that I've seen is " keyboard instruments".)

I've seen this clue in The Sunday Times and The Times.

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