The most common and most likely answer to this crossword clue is the 4 letter word BOCK
I've seen this crossword clue in The Guardian
B
O
C
K
Buddy explains!

The straight part
We know that "drink in Bonn" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drink in bonn = bock.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 BOCK mean?
BOCK noun- A strong dark beer brewed in the fall and aged through the winter for spring consumption.