Mum, lost for words (10)
I believe the answer is:
speechless
This clue is a double definition.
'mum' is the first definition.
(thesaurus)
'lost for words' is the second definition.
(I've seen this before)
(Other definitions for speechless that I've seen before include "Lost for words" , "In shock" , "Struck dumb" , "being forced to improvise?" , "Unable to say anything" .)