Plant one in weedless arrangement (9)
I believe the answer is:
edelweiss
'plant' is the definition.
(edelweiss is a kind of plant)
'one in weedless arrangement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'arrangement' indicates an anagram.
'weedless' is an anagram of 'edelwess'.
'i' placed within 'edelwess' is 'EDELWEISS'.
(Other definitions for edelweiss that I've seen before include "Sound of Music plant" , "flower as national symbol" , "I weed less when this flower is around" , "White-flowered alpine plant" , "Alpine plant, celebrated in song" .)