Extreme foolishness (6)
I believe the answer is:
idiocy
'extreme foolishness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for idiocy that I've seen before include "Predatory seabird" , "Extreme foolishness" , "Imbecility" , "Foolishness, asininity" , "Stupid act" .)