Faithless (8)
I believe the answer is:
disloyal
'faithless' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for disloyal that I've seen before include "Violating trust" , "Not faithful, treacherous" , "Traitorous, unfaithful" , "Deserting one's allegiance" , "Unfaithful or unpatriotic" .)