Turned into nice poisonous alkaloid! (8)
I believe the answer is:
nicotine
'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.
'into'+'nice'='intonice'
'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" .)