Reddish-brown dye (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">reddish-brown dye</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)