Plant in peat, so it develops (10)
I believe the answer is:
poinsettia
'plant' is the definition.
The answer is an example.
'in peat so it develops' is the wordplay.
'develops' indicates an anagram.
'in'+'peat'+'so'+'it'='inpeatsoit'
'inpeatsoit' is an anagram of '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" .)