Make sinner thinner (4)
I believe the answer is:
lisp
This could be a double definition.
'make' is the first definition.
The definition and answer can be both to do with communicating as well as being verbs in their base form.
Perhaps you can see an association between them that I can't see?
'sinner thinner' is the second definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
Can you help me to learn more?
(Other definitions for lisp that I've seen before include "Speak with defective sibilants" , "Slip into speech defect" , "Slight speech impediment" , "speak imperfectly" , "Be unable to pronounce S" .)