Faithless? Don't you believe it! (6)
I believe the answer is:
untrue
'faithless?' is the definition.
(similar in meaning)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for untrue that I've seen before include "Not in accordance with fact" , "Wrong" , "like the magnetic north?" , "Incorrect or false" , "Not accurate or veracious" .)