Pronunciation slip of the lips (4)
I believe the answer is:
lisp
'pronunciation' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'slip of the lips' is the wordplay.
'slip of' indicates anagramming the letters (I've seen 'slip' mean this).
'lips' anagrammed gives 'LISP'.
Can you help me to learn more?
(Other definitions for lisp that I've seen before include "Non-standard pronunciation" , "Have difficulty pronouncing `s'" , "Thpeak like thith" , "Be unable to pronounce S" , "peech impediment" .)