Only two-thirds of digit is insensitive (4)
I believe the answer is:
numb
'is insensitive' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for numb that I've seen before include "Without sensation" , "Lacking feeling" , "Deaden" , "Unable to feel or think" , "Frozen" .)