Possible as belief is shaken (8)
I believe the answer is:
feasible
'possible' is the definition.
(similar in meaning)
'as belief is shaken' is the wordplay.
'is shaken' indicates an anagram.
'as'+'belief'='asbelief'
'asbelief' with letters rearranged gives 'FEASIBLE'.
(Other definitions for feasible that I've seen before include "Able to be effected" , "not impossible" , "Able to be done, practical" , "An idea" , "Practicable and possible" .)