Work of monumental stupidity (5)

I believe the answer is:

folly

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'stupidity' is the definition.
(I know that folly is a type of stupidity)

'work of monumental' is the wordplay.
'work' indicates an anagram.
'monumental' becomes 'lly' (I can't justify this - if you can you should give a lot more credence to this answer).
'of' is an anagram of 'fo'.
'fo'+'lly'='FOLLY'

Can you help me to learn more?

(Other definitions for folly that I've seen before include "mock Gothic building" , "Stupidity - false building" , "Absurdity" , "Foolishness" , "Fatuity" .)

I've seen this clue in The Globe and Mail.

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