The most common and most likely answer to this crossword clue is the 6 letter word DILUTE
I've seen this crossword clue in The Guardian
D
I
L
U
T
E
More clues leading to the result DILUTE
- I’d knocked over stringed instrument, showing reduced concentration
- . Weak policeman found with stolen goods, I hear
- Concentrate less with endless cacophony on instrument
- Water down some Lucozade at first in miserable diet
- Drives in reverse over instrument, lacking concentration
- Water down funnel you invert, all ends in sink
- Thin little girl with musical instrument
- Detective on instrument having diminished concentration
- Solitude not so bad? One may get weak
- With concentration reduced, I had overturned instrument
Buddy explains!

The straight part
We know that "Water down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case water down = dilute.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 DILUTE mean?
DILUTE noun- An animal having a lighter-coloured coat than is usual.
- To make thinner by adding solvent to a solution, especially by adding water.
- To weaken, especially by adding a foreign substance.
- To cause the value of individual shares or the stake of a shareholder to decrease by increasing the total number of shares.
- To become attenuated, thin, or weak.
- Having a low concentration.
- Weak; reduced in strength by dilution; diluted.
- Of an animal: having a lighter-coloured coat than is usual.