A notice about a poisonous plant (7)
I believe the answer is:
aconite
'a poisonous plant' is the definition.
(aconite is a kind of poisonous plant)
'a notice about' is the wordplay.
'about' indicates anagramming the letters.
'notice' is an anagram of 'conite'.
'a'+'conite'='ACONITE'
(Other definitions for aconite that I've seen before include "potential killer" , "Yellow-flowered plant" , "Poisonous flowering plant" , "Flower, for example monkshood or wolfsbane" , "Spring flower - poison" .)