The most common and most likely answer to this crossword clue is the 5 letter word FOLIA
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Buddy explains!
The straight part
We know that "What’ll start falling off limes in autumn?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’ll start falling off limes in autumn? = folia.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 FOLIA mean?
FOLIA noun- A leaf, especially a thin leaf or plate.
- A curve of the third order, consisting of two infinite branches having a common asymptote. The curve has a double point, and a leaf-shaped loop.