Genitals mostly involved as point of joke (3,4)
I believe the answer is:
tag line
'point of joke' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'genitals mostly involved' is the wordplay.
'mostly' means to remove the last letter.
'involved' is an anagram indicator (involved can mean confusing or complex).
'genitals' with its final letter taken away is 'genital'.
'genital' anagrammed gives 'TAG LINE'.
'as' is the link.
Can you help me to learn more?
(Other definitions for tag line that I've seen before include "Slogan" , "Catchphrase (mainly US)" , "Something memorable one might say" , "End of joke" .)