The most common and most likely answer to this crossword clue is the 9 letter word SUBATOMIC
I've seen this crossword clue in The Sunday Telegraph
S
U
B
A
T
O
M
I
C
More clues leading to the result SUBATOMIC
- But mosaic is in pieces of very small size
- Like tiny particles of mosaic, but different
- Very tiny boat with a man in charge
- Like a very small bit of mosaic, but cast
- Very small vehicle reversing by a cat, I see
- Broadcast omits a cub, extremely small
- Intricate mosaic, but very tiny
- Very tiny coat – bum is suffering!
- Very small payment to a man in charge
- Tiny portable mosaic tub
Buddy explains!

The straight part
We know that "very tiny" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very tiny = subatomic.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 SUBATOMIC mean?
SUBATOMIC adjective- Relating to particles that are constituents of the atom, or are smaller than an atom; such as proton, neutron, electron, etc.
- Relating to any length or mass that is smaller in scale than a the diameter of a hydrogen atom.