Powdered shell's evened out noxious weed (9)
I believe the answer is:
groundsel
'noxious weed' is the definition.
'groundsel' can be an answer for 'weed' (groundsel is a kind of weed). I am not certain of the 'noxious' bit.
'powdered shell's evened out' is the wordplay.
'powdered' becomes 'ground' (I can't justify this - if you can you should believe this answer much more).
'evened out' means one should take alternating letters (I've seen 'evens out' mean this (remove letters in even-numbered positions)).
The alternating letters of 'shells' are 'sel'.
'ground'+'sel'='GROUNDSEL'
Can you help me to learn more?
(Other definitions for groundsel that I've seen before include "plant, unwanted?" , "feed to the birds" , "Plant of daisy family" , "Common weed" .)