Cheating male ultimately in wild rut with lady (8)
I believe the answer is:
adultery
'cheating' is the definition.
(cheating on a partner)
'male ultimately in wild rut with lady' is the wordplay.
'ultimately' indicates one should take the final letters.
'in' indicates putting letters inside.
'wild' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
The last letter of 'male' is 'e'.
'rut'+'lady'='rutlady'
'rutlady' with letters rearranged gives 'adultry'.
'e' going into 'adultry' is 'ADULTERY'.
(Other definitions for adultery that I've seen before include "Betrayal of a partner" , "Seventh Commandment infraction" , "Extramarital bedroom antics" , "Extramarital sex" , "frowned-on relations" .)