Sodium and potassium — deadly mixture — in plant (5,4)
I believe the answer is:
naked lady
'plant' is the definition.
(naked lady is a kind of plant)
'sodium and potassium deadly mixture' is the wordplay.
'sodium' becomes 'na' (chemical symbol).
'and' means one lot of letters go next to another.
'potassium' becomes 'k' (K is the chemical symbol for potassium).
'mixture' indicates anagramming the letters.
'deadly' with letters rearranged gives 'edlady'.
'na'+'k'+'edlady'='NAKED LADY'
'in' acts as a link.
(Other definitions for naked lady that I've seen before include "Meadow saffron" , "woman lacking 1 [UNDERGARMENTS]" , "Plant" .)