Incoherent (12)
I believe the answer is:
inarticulate
'incoherent' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for inarticulate that I've seen before include "Tongue-tied" , "Unintelligible" , "incapable of clear speech" , "Not able to speak coherently" , "incoherent" .)