The most common and most likely answer to this crossword clue is the 8 letter word PONDWEED
I've seen this crossword clue in The Times
P
O
N
D
W
E
E
D
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The straight part
We know that "water plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case water plant = pondweed.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.
Crossword Answer definitions
PONDWEED noun- Any of several plants that grow in ponds or similar aquatic conditions:
- Charales, an order of green algae