Unable to express oneself clearly (12)
I believe the answer is:
inarticulate
'unable to express oneself clearly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for inarticulate that I've seen before include "incoherent" , "Not able to speak coherently" , "Unable to express oneself clearly" , "incapable of clear speech" , "Incomprehensible" .)