Attacking cherished institutions (12)
I believe the answer is:
iconoclastic
'attacking cherished institutions' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iconoclastic that I've seen before include "Attacking cherished beliefs or institutions" , "Anti-establishment" , "Image-breaking" , "Criticising cherished beliefs" , "Nonconformist" .)