The most common and most likely answer to this crossword clue is the 7 letter word OARWEED
I've seen this crossword clue in The Times
O
A
R
W
E
E
D
Buddy explains!
The straight part
We know that "Plant that’s simple" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant that’s simple = oarweed.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 OARWEED mean?
OARWEED noun- A brown alga, Laminaria digitata, found on exposed shores