Brown is more insensitive losing head (5)
I believe the answer is:
umber
'brown' is the definition.
(umber is a kind of brown)
'more insensitive losing head' is the wordplay.
'more insensitive' becomes 'number' ('numb' can be a synonym of 'insensitive').
'losing head' means to remove the first letter.
'number' with its initial letter taken off is 'UMBER'.
'is' acts as a link.
(Other definitions for umber that I've seen before include "Earthy colour" , "Yellowish-brown pigment" , "Brown earthy mineral" , "Dark to reddish brown" , "Brownish pigment" .)