Beginning to scratch bum cheek (4)

I believe the answer is:

sass

'cheek' is the definition.
(both can mean rudeness or insolence)

'beginning to scratch bum' is the wordplay.
'beginning to' says to take the initial letters.
'bum' becomes 'ass' (synonyms).
The initial letter of 'scratch' is 's'.
's'+'ass'='SASS'

(Another definition for sass that I've seen is " Cheek".)

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