Stab in the back (6)
I believe the answer is:
betray
'stab in the back' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for betray that I've seen before include "Expose" , "Be disloyal to like Judas" , "'Deceive, break trust (6)'" , "Be disloyal to, abuse trust" , "grass" .)