Kiss inside turning into poison (5)
I believe the answer is:
toxin
'poison' is the definition.
(I know that toxin is a type of poison)
'kiss inside turning into' is the wordplay.
'kiss' becomes 'x' (X can represent a kiss).
'inside' is an insertion indicator.
'turning' indicates an anagram.
'into' is an anagram of 'toin'.
'x' going into 'toin' is 'TOXIN'.
(Other definitions for toxin that I've seen before include "Form of poison" , "Poison of organic origin" , "Poison, venom" , "Natural poison" , "Poisonous substance" .)