A drug almost does for the leading lady (6)
I believe the answer is:
heroin
'a drug' is the definition.
(I know that Heroin is a opioid)
'almost does for the leading lady' is the wordplay.
I cannot quite see how this works, but
'leading' could be 'in' (in government) and 'in' is found in the answer.
'lady' could be 'her' (her refers to a woman) and 'her' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for heroin that I've seen before include "Hard drug" , "junk" , "Highly addictive morphine derivative" , "Type of drug" , "Class A drug" .)