Betrayal of one's country (7)
I believe the answer is:
treason
'betrayal of one's country' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for treason that I've seen before include "One rats (anag)" , "Betrayal of one's nation" , "Deliberate act of betrayal" , "LEse-majestE" , "One's art? (anag)" .)