Changing belief around when possible (8)
I believe the answer is:
feasible
'possible' is the definition.
(I've seen this before)
'changing belief around when' is the wordplay.
'changing' indicates anagramming the letters.
'around' means one lot of letters goes inside another.
'when' becomes 'as'.
'belief' with letters rearranged gives 'feible'.
'feible' enclosing 'as' is 'FEASIBLE'.
(Other definitions for feasible that I've seen before include "workable" , "Achievable" , "Practicable and possible" , "not impossible" , "Capable of being done" .)