Plant in peat, so it develops (10)
I believe the answer is:
Here is my best explanation:
'plant' is the definition.
The answer is an example.
'in peat so it develops' is the subsidiary indication.
'develops' indicates an anagram.
'inpeatsoit' anagrammed gives 'poinsettia'.
(Other definitions for poinsettia that I've seen before include "poisonous bush", "American plant", "Plant with showy bracts, usu. red, pink or white", "bloom", "Ornamental house plant", "Flower popular at Christmas", "Shrub often given at Christmas" and "House plant, popular at Christmas -- tiniest OAP (anag)".)