It's possible a belief's being shaken (8)
I believe the answer is:
feasible
'it's possible' is the definition.
(I've seen this before)
'a belief's being shaken' is the wordplay.
'being shaken' indicates an anagram.
'a'+'beliefs'='abeliefs'
'abeliefs' anagrammed gives 'FEASIBLE'.
(Other definitions for feasible that I've seen before include "Practicable and possible" , "Easily possible to do" , "not impossible" , "Base life (anag.)" , "Able to be effected" .)