Not as busy in practice (6)

I believe the answer is:

lesson

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'practice' is the definition.
(thesaurus)

'not as busy' is the wordplay.
'not' becomes 'less' (I've seen this in other clues).
'as busy' becomes 'on' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'less'+'on'='LESSON'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for lesson that I've seen before include "Piece of instruction" , "Severe reproof" , "Church reading" , "Moral" , "What teacher provides" .)

I've seen this clue in The Telegraph.

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