Ridiculous poseur, dancing clutching Bill (12)
I believe the answer is:
preposterous
'ridiculous' is the definition.
(I know that ridiculous can be written as preposterous)
'poseur dancing clutching bill' is the wordplay.
'dancing' indicates an anagram.
'clutching' is an insertion indicator.
'bill' becomes 'poster' (synonyms).
'poseur' with letters rearranged gives 'preous'.
'preous' placed around 'poster' is 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Utterly absurd" , "Quite absurd" , "Utterly ridiculous" , "Ludicrous" , "Crazy" .)