The most common and most likely answer to this crossword clue is the 9 letter word PLANTAINS
I've seen this crossword clue in Financial Times
P
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A
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T
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Buddy explains!
The straight part
We know that "They grow on trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they grow on trees = plantains.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 PLANTAINS mean?
PLANTAINS noun- A plant of the genus Plantago, with a rosette of sessile leaves about 10 cm long with a narrow part instead of a petiole, and with a spike inflorescence with the flower spacing varying widely among the species. See also psyllium.
- A plant in the genus Musa, the genus that includes banana, but with lower sugar content than banana.
- The fruit of the plant, usually cooked before eating and used like potatoes.