The most common and most likely answer to this crossword clue is the 9 letter word RAT-ARSED
I've seen this crossword clue in The Guardian
R
A
T
-
A
R
S
E
D
Buddy explains!
The straight part
We know that "in drink" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in drink = rat-arsed.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RAT-ARSED mean?
RAT-ARSED adjective- Extremely drunk.