The most common and most likely answer to this crossword clue is the 13 letter word TEENSY-WEENSY
I've seen this crossword clue in The Times
T
E
E
N
S
Y
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W
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Y
Buddy explains!

The straight part
We know that "Very small" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very small = teensy-weensy.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 TEENSY-WEENSY mean?
TEENSY-WEENSY adjective- Very small; tiny; minuscule.