Infidelity snarled at rudely (8)
I believe the answer is:
adultery
'infidelity' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for adultery that I've seen before include "cheating" , "Moral lapse" , "Extramarital bedroom antics" , "excluding union involvement?" , "frowned-on relations" .)