Knocking sound king at a touch (7)

I believe the answer is:

ratatat

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'knocking sound' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'king at a touch' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'touch' becomes 'tat' (I am not sure about this - if you are sure you should believe this answer much more).
'r'+'at'+'a'+'tat'='RATATAT'

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I've seen this clue in the Sydney Morning Herald.

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