Have trouble saying "s" (4)
I believe the answer is:
lisp
'have trouble saying s' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lisp that I've seen before include "Speak with defective sibilants" , "Slip into speech defect" , "speak imperfectly" , "Have difficulty pronouncing `s'" , "Slight speech impediment" .)