Turned into nice poisonous alkaloid! (8)
I believe the answer is:
'poisonous alkaloid' is the definition.
'nicotine' can be an answer for 'alkaloid' (nicotine is a kind of alkaloid). I'm unsure of the 'poisonous' bit.
'turned into nice' is the wordplay.
'turned' indicates anagramming the letters.
'intonice' anagrammed gives 'NICOTINE'.
Can you help me to learn more?
(Other definitions for nicotine that I've seen before include "Not nice, I change to cigarette substance" , "Active constituent in tobacco" , "Poisonous substance in tobacco" , "Drug in tobacco" , "Once in it, strangely, one smokes it" .)