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High frequency range, say, penetrating mum — is sore in the ears (9)

Ross

I believe the answer is:

megahertz

'high frequency range' is the definition.

'say penetrating mum is sore in the ears' is the wordplay.
'say' becomes 'eg'.
'penetrating' indicates putting letters inside.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'is sore in the ears' becomes 'hertz' (I can't explain this - if you can you should believe this answer much more).
'eg' inserted into 'ma' is 'mega'.
'mega'+'hertz'='MEGAHERTZ'

(Another definition for megahertz that I've seen is " MHz in full".)

I've seen this clue in The Guardian.
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