Absurd purpose store devised (12)
I believe the answer is:
preposterous
'absurd' is the definition.
('preposterous' can be a synonym of 'absurd')
'purpose store devised' is the wordplay.
'devised' is an anagram indicator.
'purpose'+'store'='purposestore'
'purposestore' anagrammed gives 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Crazy" , "Unreasonable - fatuous" , "Utterly absurd" , "Utterly ridiculous" , "Quite absurd" .)