Rebellious businessman is departing from agenda, initially because of a complete lack of enthusiasm (8)

I believe the answer is:

numbness

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'lack of enthusiasm' is the definition.
'numbness' can be an answer for 'lack' (I have seen 'Lack of feeling ' mean 'numbness' so perhaps 'lack' could also mean 'numbness'). I am unsure of the 'of enthusiasm' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for numbness that I've seen is " Lack of feeling".)

I've seen this clue in The Irish Times.

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