Drunk, heartless, is internally suffering (6)
I believe the answer is:
misery
'suffering' is the definition.
(misery is suffering or torment)
'drunk heartless is internally' is the wordplay.
'drunk' becomes 'merry' (merry can mean cheerfully drunk).
'heartless' says to hollow out the word (remove centre letters).
'internally' means one lot of letters goes inside another.
'merry' with its centre taken out is 'mery'.
'mery' enclosing 'is' is 'MISERY'.
(Other definitions for misery that I've seen before include "Wretched conditions" , "Intense unhappiness" , "Great suffering" , "Feeling of great unhappiness" , "Party-pooper" .)