In kitchen ban every herb that could be lethal? (7)
I believe the answer is:
henbane
'herb that could be lethal?' is the definition.
(poisonous herb)
'in kitchen ban every' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HENBANE' is hidden within 'kitchen ban every'.
(Other definitions for henbane that I've seen before include "poison" , "Poisonous plant" , "Poisoner" .)