Lost for words (10)
'lost for words' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for speechless that I've seen before include "Unable to say anything" , "In shock" , "Dumbstruck" , "Dumbfounded" , "Lost for words" .)
(Other definitions for inarticulate that I've seen before include "Unable to speak clearly" , "Not clearly expressed" , "Incomprehensible" , "I taunt a relic (anag.)" , "incoherent" .)