Chaos in opium den, man misbehaving (11)
I believe the answer is:
pandemonium
'chaos' is the definition.
('pandemonium' can be a synonym of 'chaos')
'opium den man misbehaving' is the wordplay.
'misbehaving' indicates an anagram.
'opium'+'den'+'man'='opiumdenman'
'opiumdenman' is an anagram of 'PANDEMONIUM'.
'in' is the link.
(Other definitions for pandemonium that I've seen before include "State of chaos" , "Uproar, wild and noisy confusion" , "Chaos and confusion" , "Bedlam" , "Tumultuous uproar" .)