The most common and most likely answer to this crossword clue is the 7 letter word HYPERON
I've seen this crossword clue in The Guardian
H
Y
P
E
R
O
N
Buddy explains!

The straight part
We know that "Very small thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very small thing = hyperon.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 HYPERON mean?
HYPERON noun- Any baryon with a non-zero strangeness (i.e., whose composition includes one or more strange or anti-strange quarks).