The most common and most likely answer to this crossword clue is the 8 letter word PALMETTO
I've seen this crossword clue in The Guardian
P
A
L
M
E
T
T
O
Buddy explains!

The straight part
We know that "semi-tropical tree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case semi-tropical tree = palmetto.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 PALMETTO mean?
PALMETTO noun- Any member of either of two closely related genera of New World palms, of the family Arecaceae:
- A hat made of palmetto leaves.
- A native or resident of the US state of South Carolina.