Inane behavior (6)
I believe the answer is:
idiocy
'inane behavior' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for idiocy that I've seen before include "Stupid behaviour or talk" , "Foolishness, asininity" , "Extreme silliness" , "Imbecility" , "Complete folly" .)