Opiate isn't derived from this plant (10)
I believe the answer is:
poinsettia
'this plant' is the definition.
The definition and answer can be both plants as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'opiate isn't derived from' is the wordplay.
'derived from' indicates anagramming the letters.
'opiate'+'isnt'='opiateisnt'
'opiateisnt' anagrammed gives 'POINSETTIA'.
Can you help me to learn more?
(Other definitions for poinsettia that I've seen before include "American plant" , "poisonous bush" , "Shrub often given at Christmas" , "Plant with showy bracts, usu. red, pink or white" , "House plant, popular at Christmas -- tiniest OAP (anag)" .)