The most common and most likely answer to this crossword clue is the 9 letter word SPRITZERS
I've seen this crossword clue in The Guardian
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More clues leading to the result SPRITZERS
Buddy explains!

The straight part
We know that "watered-down drinks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case watered-down drinks = spritzers.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 SPRITZERS mean?
SPRITZERS noun- A chilled long drink, made from white wine and soda water.
- A windscreen washer.
- A spray bottle.