The most common and most likely answer to this crossword clue is the 4 plus 5 letter word NEAT DRINK
I've seen this crossword clue in The Guardian
N
E
A
T
D
R
I
N
K
Buddy explains!
The straight part
We know that "On the rocks? Not in this case" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on the rocks? not in this case = neat drink.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.