About to take the plunge perhaps, mixing nicotine with heroin (2,4,3)
I believe the answer is:
on thin ice
'about to take the plunge perhaps' is the definition.
(as in about to plunge into water)
'mixing nicotine with heroin' is the wordplay.
'mixing' indicates anagramming the letters.
'with' indicates putting letters inside.
'heroin' becomes 'h' (synonyms).
'nicotine' with letters rearranged gives 'ontinice'.
'ontinice' going around 'h' is 'ON THIN ICE'.
(Other definitions for on thin ice that I've seen before include "In a dangerous situation" , "In deep water" , "sailing close to the wind?" , "In perilous situation - don't skate thus (2,4,3)" , "In a risky situation" .)