Marital infidelity (8)
I believe the answer is:
adultery
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">marital infidelity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)