Red dye (5)
'red dye' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for henna that I've seen before include "Local colour" , "Reddish-brown plant dye" , "hair colour?" , "shock treatment" , "Red-brown dye used for body decorations" .)
(Other definitions for cochineal that I've seen before include "Red food dye (from beetles)" , "Crimson dye from crushed insects" , "Red food colouring" , "Scarlet dye - in a cloche (anag)" , "Red colour" .)