Faithful convert cannot, without saint (8)
I believe the answer is:
constant
'faithful' is the definition.
('constant' can be similar in meaning to 'faithful')
'convert cannot without saint' is the wordplay.
'convert' indicates an anagram (I've seen this in other clues).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'saint' becomes 'st'.
'cannot' with letters rearranged gives 'conant'.
'conant' enclosing 'st' is 'CONSTANT'.
(Other definitions for constant that I've seen before include "sustained" , "Remaining the same" , "Unwavering" , "without changes" , "Never-ending" .)