Unthinkable treatment of Peru’s poorest! (12)
I believe the answer is:
preposterous
'unthinkable' is the definition.
(thesaurus)
'treatment of peru's poorest' is the wordplay.
'treatment of' indicates an anagram (letters treated or processed).
'perus'+'poorest'='peruspoorest'
'peruspoorest' anagrammed gives 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Ludicrous" , "Laughable" , "Quite absurd" , "Utterly absurd" , "Unreasonable - fatuous" .)