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