Drunk on the hard stuff, is in a very good mood (4,7)
I believe the answer is:
high spirits
'a very good mood' is the definition.
'high spirits' can be an answer for 'mood' (I have seen 'Cheerful mood' mean 'high spirits' so perhaps 'mood' could also mean 'high spirits'). I am not certain of the 'a very good' bit.
'drunk on the hard stuff' is the wordplay.
'drunk' becomes 'high' (I've seen this before).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'the hard stuff' becomes 'spirits' (I can't explain this - if you can you should believe this answer much more).
'high'+'spirits'='HIGH SPIRITS'
'is in' acts as a link.
Can you help me to learn more?
(Other definitions for high spirits that I've seen before include "Elated frame of mind" , "Cheerful mood" .)