Belief is lacking in discretion, possibly (8)
I believe the answer is:
doctrine
'belief' is the definition.
(doctrine is a kind of belief)
'is lacking in discretion possibly' is the wordplay.
'lacking in' suggests deleting specific letters.
'possibly' is an anagram indicator.
'discretion' with 'is' removed is 'dcretion'.
'dcretion' anagrammed gives 'DOCTRINE'.
(Other definitions for doctrine that I've seen before include "No credit given to belief" , "No credit given for set of beliefs" , "Set of beliefs taught by church" , "Church's policy" , "Guiding principle" .)