The most common and most likely answer to this crossword clue is the 5 letter word MESON
I've seen this crossword clue in The Times
M
E
S
O
N
Buddy explains!

The straight part
We know that "tiny thing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tiny thing = meson.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 MESON mean?
MESON noun- (rare outside entomology) The mesial plane dividing the body into similar right and left halves.
- A member of a group of subatomic particles having a mass intermediate between electrons and protons. (The most easily detected mesons fit this definition.)
- (now specifically) An elementary particle that is composed of a quark and an antiquark, such as a kaon or pion. (Mesons composed of rarer quarks are much heavier.)