Wretched poseur holding placard that’s absurd (12)
I believe the answer is:
'absurd' is the definition.
(I know that absurd can be written as preposterous)
'wretched poseur holding placard' is the wordplay.
'wretched' is an anagram indicator.
'holding' means one lot of letters goes inside another.
'placard' becomes 'poster' (synonyms).
'poseur' is an anagram of 'preous'.
'preous' enclosing 'poster' is 'PREPOSTEROUS'.
'that's' acts as a link.
(Other definitions for preposterous that I've seen before include "Quite absurd" , "Crazy" , "Utterly ridiculous" , "Unreasonable - fatuous" , "Utterly absurd" .)