The most common and most likely answer to this crossword clue is the 6 plus 3 plus 9 letter word SPLICE THE MAINBRACE
I've seen this crossword clue in The Guardian
S
P
L
I
C
E
T
H
E
M
A
I
N
B
R
A
C
E
Buddy explains!
The straight part
We know that "have a drink" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case have a drink = splice the mainbrace.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.