The most common and most likely answer to this crossword clue is the 2 plus 3 plus 5 letter word ON THE ROCKS
I've seen this crossword clue in The Times
O
N
T
H
E
R
O
C
K
S
More clues leading to the result ON THE ROCKS
Buddy explains!
The straight part
We know that "Whisky served in this way/ruined" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whisky served in this way/ruined = on the rocks.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.