Imperfect speech (4)
I believe the answer is:
lisp
'imperfect speech' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lisp that I've seen before include "Be unable to pronounce S" , "Have difficulty pronouncing `s'" , "Slight speech impediment" , "Talk like thith" , "Non-standard pronunciation" .)