Changing a belief's possible (8)
I believe the answer is:
feasible
'possible' is the definition.
(I've seen this before)
'changing a belief's' is the wordplay.
'changing' is an anagram indicator.
'a'+'beliefs'='abeliefs'
'abeliefs' is an anagram of 'FEASIBLE'.
(Other definitions for feasible that I've seen before include "Practicable and possible" , "not impossible" , "Capable of being done" , "Achievable" , "An idea" .)