Daffy Duck trademark (4)
I believe the answer is:
lisp
'daffy duck trademark' 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 "Have difficulty pronouncing `s'" , "Talk like thith" , "Thpeak like thith" , "Slight speech impediment" , "Speak with defective sibilants" .)