His missus being unfaithful makes him horny (7)
I believe the answer is:
cuckold
This might be a double definition.
'his missus' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'unfaithful makes him horny' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'being' is the link.
(Other definitions for cuckold that I've seen before include "Husband with an unfaithful wife" , "Man whose wife has been adulterous" , "fool of a husband" , "Betrayed husband" , "One cheated" .)