Make numb; muffle (6)
I believe the answer is:
deaden
I believe this clue is a double definition.
'make numb' is the first definition.
(I've seen this in another clue)
'muffle' is the second definition.
(synonyms)
(Other definitions for deaden that I've seen before include "Muffle, make muted" , "Kill" , "dull" , "Take life" , "Make numb, deprive of feeling" .)