Layer not applicable in hair dye (5)
I believe the answer is:
henna
'hair dye' is the definition.
(henna is a kind of hair dye)
'layer not applicable' is the wordplay.
'layer' becomes 'hen' (I've seen this before).
'not applicable' becomes 'na' (this might be a standard abbreviation which I don't know about).
'hen'+'na'='HENNA'
'in' is the link.
Can you help me to learn more?
(Other definitions for henna that I've seen before include "Skin and hair dye" , "Red-brown plant dye" , "Dye used in skin decoration" , "Shrub used for reddish-brown dye" , "(Shrub producing) reddish dyestuff" .)