Jamming with Lennon or Clapton, initially the greatest (7)

I believe the answer is:

locking

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'jamming' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?

'lennon or clapton initially the greatest' is the wordplay.
'initially' suggests taking the first letters.
'the greatest' becomes 'king' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The initial letters of 'lennon or clapton' is 'loc'.
'loc'+'king'='LOCKING'

'with' is the link.

Can you help me to learn more?

(Another definition for locking that I've seen is " Securing".)

I've seen this clue in The Guardian.

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