The most common and most likely answer to this crossword clue is the 9 letter word MEASLIEST
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More clues leading to the result MEASLIEST
Buddy explains!

The straight part
We know that "Extremely small" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely small = measliest.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 MEASLIEST mean?
MEASLIEST adjective- Particularly of pigs or pork: infected with larval tapeworms or trichinae (parasitic roundworms).
- Of a person: infected with measles.
- Small (especially contemptibly small) in amount.