Reddish-brown dye (5)
I believe the answer is:
'reddish-brown dye' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for henna that I've seen before include "Skin and hair dye" , "Shrub used for cosmetic colouring" , "shock treatment" , "Local colour" , "Hair/skin dye" .)