Music may defeat copper, say (6,5)

I believe the answer is:

thrash metal

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

'music may defeat copper say' is the definition.
I can't judge whether this definition defines the answer.

'defeat copper say' is the wordplay.
'defeat' becomes 'thrash' (I've seen this before).
'copper say' becomes 'metal' (copper is a kind of metal).
'thrash'+'metal'='THRASH METAL'

Can you help me to learn more?

(Another definition for thrash metal that I've seen is " Type of music".)

I've seen this clue in The Mirror.

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