Ludicrous poser upset or disturbed (12)
I believe the answer is:
preposterous
'ludicrous' is the definition.
('preposterous' can be a synonym of 'ludicrous')
'poser upset or disturbed' is the wordplay.
'disturbed' is an anagram indicator.
'poser'+'upset'+'or'='poserupsetor'
'poserupsetor' is an anagram of 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Ludicrous" , "Quite absurd" , "Utterly absurd" , "Crazy" , "Laughable" .)