Once it is compounded with a poison (7)
I believe the answer is:
aconite
'poison' is the definition.
(I've seen this before)
'once it is compounded with a' is the wordplay.
'is compounded with' indicates anagramming the letters.
'once'+'it'='onceit'
'onceit' mixed up with 'a' can make 'ACONITE'.
(Other definitions for aconite that I've seen before include "Flower, for example monkshood or wolfsbane" , "Yellow-flowered plant" , "potential killer" , "Poisonous flowering plant" , "Spring flower - poison" .)