Criticise medium on dreadful chaos (11)
I believe the answer is:
pandemonium
'chaos' is the definition.
(I know that chaos can be written as pandemonium)
'criticise medium on dreadful' is the wordplay.
'criticise' becomes 'pan' (informal term meaning to criticise heavily).
'dreadful' is an anagram indicator.
'medium'+'on'='mediumon'
'mediumon' is an anagram of 'demonium'.
'pan'+'demonium'='PANDEMONIUM'
(Other definitions for pandemonium that I've seen before include "Tumultuous uproar" , "Chaos and confusion" , "Bedlam" , "Uproar, wild and noisy confusion" , "mayhem" .)