Speak like Sylvester (4)
I believe the answer is:
lisp
'speak like sylvester' 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 "Talk like thith" , "Slip into speech defect" , "Have difficulty pronouncing `s'" , "Speak with defective sibilants" , "Non-standard pronunciation" .)