Plants start to bloom in joy (9)
I believe the answer is:
euphorbia
'plants' is the definition.
(euphorbia is the genus including spurge plants)
'start to bloom in joy' is the wordplay.
'start to' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'joy' becomes 'euphoria' (I've seen this before).
The first letter of 'bloom' is 'b'.
'b' put into 'euphoria' is 'EUPHORBIA'.
(Other definitions for euphorbia that I've seen before include "poinsettia" , "Plant of the spurge genus" , "flower" , "shrub" , "flowering plant" .)