Ridiculous trousers Pope gets tailored (12)
I believe the answer is:
preposterous
'ridiculous' is the definition.
(synonyms)
'trousers pope gets tailored' is the wordplay.
'gets tailored' is an anagram indicator.
'trousers'+'pope'='trouserspope'
'trouserspope' with letters rearranged gives 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Quite absurd" , "Ludicrous" , "Crazy" , "Laughable" , "Utterly absurd" .)