Unable to speak (10)
There are a few possibilities for this:
speechless
'unable to' is the definition.
(cannot)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cant that I've seen before include "Incline, slope" , "Not able to" , "Special jargon of a group" , "tilt" , "Type 16 [OF SPEECH]" .)
'unable to speak' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mute that I've seen before include "Not pronounced" , "Silent, not talking" , "Silence" , "Unspeaking" , "having no voice" .)
(Other definitions for speechless that I've seen before include "Lost for words" , "In shock" , "being forced to improvise?" , "Unable to say anything" , "Struck dumb" .)