Extreme foolishness, but with any luck almost all will reform (6)

I believe the answer is:

lunacy

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'extreme foolishness' is the definition.
'lunacy' can be an answer for 'foolishness' (thesaurus). I'm not certain of the 'extreme' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for lunacy that I've seen before include "Madness, insanity" , "Tomfoolery" , "Extreme folly, insanity" , "Craziness, madness" , "State of madness" .)

I've seen this clue in the Evening Standard.

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