The most common and most likely answer to this crossword clue is the 6 letter word SEROUS
I've seen this crossword clue in The Daily Telegraph
S
E
R
O
U
S
Buddy explains!
The straight part
We know that "containing watery liquid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case containing watery liquid = serous.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 SEROUS mean?
SEROUS adjective- Containing, secreting, or resembling serum; watery; a fluid or discharge that is pale yellow and transparent, usually representing something of a benign nature. (This contrasts with the term sanguine, which means blood-tinged and usually harmful.)