The most common and most likely answer to this crossword clue is the 7 letter word TAGETES
T
A
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The straight part
We know that "S. American plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case s. american plant = tagetes.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 TAGETES mean?
TAGETES noun- Any plant of the marigold (Tagetes) family.
- Any plant of the marigold (Tagetes) family.