The most common and most likely answer to this crossword clue is the 9 letter word DEUTERONS
I've seen this crossword clue in The Daily Telegraph
D
E
U
T
E
R
O
N
S
Buddy explains!

The straight part
We know that "Very small things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very small things = deuterons.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 DEUTERONS mean?
DEUTERONS noun- The atomic nucleus of a deuterium atom, consisting of a proton and a neutron