RHS chum may net hybridised flowering plant (13)
I believe the answer is:
chrysanthemum
'flowering plant' is the definition.
(chrysanthemum is a kind of flowering plant)
'rhs chum may net hybridised' is the wordplay.
'hybridised' indicates anagramming the letters (letters hybridised or mixed up).
'rhs'+'chum'+'may'+'net'='rhschummaynet'
'rhschummaynet' is an anagram of 'CHRYSANTHEMUM'.
(Other definitions for chrysanthemum that I've seen before include "Genus of flowering plants related to the daisy family" , "Ornamental flower" , "Flower depicted on Japanese Imperial flag" , "Flower, literally golden one" , "Popular garden flower" .)