Act of deliberate betrayal (7)
I believe the answer is:
treason
'act of deliberate betrayal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for treason that I've seen before include "Betrayal (of one's country)" , "Perfidy" , "Lese-majesty" , "One rats (anag)" , "Disloyalty (to one's country)" .)