International organisation intricately decorated outside? Madness! (6)
I believe the answer is:
lunacy
'madness' is the definition.
('lunacy' can be a synonym of 'madness')
'international organisation intricately decorated outside?' is the wordplay.
'international organisation' becomes 'un'.
'intricately decorated' becomes 'lacy'.
'outside?' is an insertion indicator.
'un' inserted within 'lacy' is 'LUNACY'.
(Other definitions for lunacy that I've seen before include "idiotic act" , "Tomfoolery" , "Craziness, madness" , "Extreme folly, insanity" , "Madness, insanity" .)