Ornamental flower (3)
I believe the answer is:
mum
'ornamental flower' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mum that I've seen before include "Silence" , "Mother (colloq.)" , "Tight-lipped" , "Quiet; relative" , "Mother - she's the word for secrecy" .)