Poisonous plant — it is buried in a geometric shape (7)
I believe the answer is:
aconite
'poisonous plant' is the definition.
(aconite is a kind of poisonous plant)
'it is buried in a geometric shape' is the wordplay.
'is buried in' indicates putting letters inside.
'geometric shape' becomes 'cone' (cone is a kind of shape. I am not sure about the 'geometric' bit.**).
'a'+'cone'='acone'
'it' inserted within 'acone' is 'ACONITE'.
Can you help me to learn more?
(Other definitions for aconite that I've seen before include "Spring flower - poison" , "potential killer" , "Flower, for example monkshood or wolfsbane" , "Poisonous flowering plant" , "Yellow-flowered plant" .)