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 has a value of one in some card games).
'is about' indicates putting letters inside.
'to mix' is an anagram indicator.
'into' anagrammed gives 'onit'.
'ace' placed around '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" .)