Insane about the last drink (4)
I believe the answer is:
mead
'drink' is the definition.
(mead is a kind of drink)
'insane about the last' is the wordplay.
'insane' becomes 'mad' ('mad' can be a synonym of 'insane').
'about' means one lot of letters goes inside another.
'last' suggests the final letters.
The final letter of 'the' is 'e'.
'mad' going around 'e' is 'MEAD'.
(Other definitions for mead that I've seen before include "Honey-based alcoholic drink" , "The poetic field" , "Cooked joint" , "Male shop worker" , "Alcoholic drink made with honey" .)