Poison one is about to mix into (7)
I believe the answer is:
aconite
'poison' is the definition.
(I've seen this before)
'one is about to mix into' is the wordplay.
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'is about' is an insertion indicator.
'to mix' is an anagram indicator.
'into' with letters rearranged gives 'onit'.
'ace' enclosing 'onit' is 'ACONITE'.
(Other definitions for aconite that I've seen before include "Spring flower - poison" , "Flower, for example monkshood or wolfsbane" , "potential killer" , "Poisonous flowering plant" , "Yellow-flowered plant" .)