Unwise undertaking (5)
I believe the answer is:
folly
'unwise undertaking' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for folly that I've seen before include "mock Gothic building" , "Foolishness" , "Fatuity" , "'Madness, stupidity (5)'" , "Silliness - stupid mistake" .)