Off heroin? On it, you are quite arrogant (4,5)
I believe the answer is:
high horse
'it you are quite arrogant' is the definition.
The definition suggests a singular noun which matches the answer.
'off heroin? on' is the wordplay.
'off' becomes 'high' (both can mean rotten).
'heroin?' becomes 'horse' (slang term for heroin).
'on' means one lot of letters go next to another.
'high'+'horse'='HIGH HORSE'
Can you help me to learn more?
(Other definitions for high horse that I've seen before include "Lofty steed for haughty person" , "on which one is over-assertive" , "on it one is arrogant" .)