Its use may be said to be dying (5)
I believe the answer is:
henna
'be dying' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for henna that I've seen before include "A shrub, its red dye" , "hair colouring" , "Red-brown dye used for body decorations" , "Reddish dye for hair etc" , "natural colour?" .)