Stupid behaviour or action (6)
I believe the answer is:
idiocy
This clue is a double definition.
'stupid behaviour' is the first definition.
(I've seen this before)
'action' is the second definition.
(I have seen 'Foolish action ' mean 'idiocy' so perhaps 'action' could also mean 'idiocy')
(Other definitions for idiocy that I've seen before include "Foolish act" , "Great folly" , "Imbecility" , "Foolishness, asininity" , "Stupid behaviour or talk" .)