The most common and most likely answer to this crossword clue is the 4 letter word KELP
I've seen this crossword clue in Independent
K
E
L
P
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The straight part
We know that "Seaweed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seaweed = kelp.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 KELP mean?
KELP noun- Any of several large brown seaweeds (order Laminariales).
- The calcined ashes of seaweed, formerly used in glass and iodine manufacture.