Where one feels superior, drugged by heroin (4,5)
I believe the answer is:
high horse
'where one feels' is the definition.
The definition suggests a singular noun which matches the answer.
'superior drugged by heroin' is the wordplay.
I cannot quite understand how this works, but
'superior' could be 'high' (associated in meaning) and 'high' is located in the answer.
'heroin' could be 'horse' (slang term for heroin) and 'horse' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for high horse that I've seen before include "on which one is over-assertive" , "Lofty steed for haughty person" , "on it one is arrogant" .)