Marital infidelity (8)
I believe the answer is:
adultery
'marital infidelity' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for adultery that I've seen before include "The act of marital infidelity" , "frowned-on relations" , "Act of infidelity to spouse" , "Extramarital sex" , "Act of infidelity by husband or wife" .)