Decline notes on peer group (12)

I believe the answer is:

degeneration

I'm a little stuck... Click here to teach me more about this clue!

'decline' is the definition.
(I've seen this before)

'notes on peer group' is the wordplay.
'notes on' becomes 'de' (two musical notes. I am not sure about the 'on' bit.).
'peer group' becomes 'generation' (I've seen this before).
'de'+'generation'='DEGENERATION'

Can you help me to learn more?

(Other definitions for degeneration that I've seen before include "Atrophy" , "Decadence" , "The decline" , "Degradation" .)

I've seen this clue in The Daily Express.

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