Crossword Genius

Only two-thirds of digit is insensitive (4)

Ross

I believe the answer is:

numb

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'is insensitive' is the definition.
(synonyms)

I can't explain the remainder of the clue.

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(Other definitions for numb that I've seen before include "Without sensation" , "Lacking feeling" , "Deaden" , "Unable to feel or think" , "Frozen" .)

I've seen this clue in The Daily Express.
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