Knocking appointment in circle (7)

I believe the answer is:

rapping

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'knocking' is the definition.
('rap' can be a synonym of 'knock')

'appointment in circle' is the wordplay.
'appointment' becomes 'app' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'circle' becomes 'ring' (synonyms).
'app' placed into 'ring' is 'RAPPING'.

Can you help me to learn more?

(Another definition for rapping that I've seen is " Striking".)

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

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