Sigmatism (4)
I believe the answer is:
lisp
'sigmatism' is the definition.
(I know that sigmatism can be written as lisp)
This is the entire clue.
(Other definitions for lisp that I've seen before include "Non-standard pronunciation" , "Have difficulty pronouncing `s'" , "Be unable to pronounce S" , "peech impediment" , "Talk like thith" .)