Work of monumental stupidity (5)
I believe the answer is:
'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'.
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" .)