The most common and most likely answer to this crossword clue is the 7 letter word THINNED
I've seen this crossword clue in The Daily Telegraph
T
H
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More clues leading to the result THINNED
Buddy explains!

The straight part
We know that "diluted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diluted = thinned.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 THINNED mean?
THINNED verb- To make thin or thinner.
- To become thin or thinner.
- To dilute.
- To remove some plants or parts of plants in order to improve the growth of what remains.