The most common and most likely answer to this crossword clue is the 11 letter word MEADOWSWEET
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More clues leading to the result MEADOWSWEET
Buddy explains!

The straight part
We know that "Weed – somewhat unkempt, striking height?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case weed – somewhat unkempt, striking height? = meadowsweet.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 MEADOWSWEET mean?
MEADOWSWEET noun- A Eurasian perennial flowering plant of Rosaceae family, Filipendula ulmaria.
- Any plant of the genus Spiraea of the Rosaceae family, native to the temperate Northern Hemisphere and consisting of about 80-100 species of shrubs.