A purist's changed mentally (8)
I believe the answer is:
upstairs
'mentally' is the definition.
(as in not having much going on upstairs)
'a purist's changed' is the wordplay.
'changed' is an anagram indicator.
'a'+'purists'='apurists'
'apurists' anagrammed gives 'UPSTAIRS'.
(Other definitions for upstairs that I've seen before include "at the next level" , "Mentally (colloquial)" , "Above ground floor" , "On an upper storey" , "Having taken steps to rise" .)