Not faithful (8)
I believe the answer is:
disloyal
'not faithful' is the definition.
('disloyal' is the opposite to 'faithful')
This is all the clue.
(Other definitions for disloyal that I've seen before include "Not faithful, treacherous" , "Deserting allegiance or duty" , "Unfaithful or unpatriotic" , "Treasonable or treacherous" , "Deserting one's allegiance" .)