Less sensitive to quantity (6)
I believe the answer is:
number
I believe this clue is a double definition.
'less sensitive' is the first definition.
(numb means insensitive or unfeeling)
'to quantity' is the second definition.
(number is a kind of quantity)
(Other definitions for number that I've seen before include "comparatively lacking in feeling" , "theme tune?" , "42, say" , "27, say" , "1,2,3, for instance" .)