The most common and most likely answer to this crossword clue is the 8 letter word HYDRATES
I've seen this crossword clue in Financial Times
H
Y
D
R
A
T
E
S
Buddy explains!
The straight part
We know that "Adds water to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case adds water to = hydrates.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 HYDRATES mean?
HYDRATES noun- A solid compound containing or linked to water molecules.
- Water.
- To take up, consume or become linked to water.
- To drink water.
- To load data from a database record into an object's variables