Feeling of great happiness about black plant (9)
I believe the answer is:
euphorbia
'plant' is the definition.
'feeling of great happiness about black' is the wordplay.
'feeling of great happiness' becomes 'euphoria'.
'about' indicates putting letters inside.
'black' becomes 'b' (as in pencil classifications).
'euphoria' placed around 'b' is 'EUPHORBIA'.
(Other definitions for euphorbia that I've seen before include "shrub" , "flowering plant" , "poinsettia" , "Plant of the spurge genus" , "flower" .)