The most common and most likely answer to this crossword clue is the 9 letter word MOLECULAR
I've seen this crossword clue in The Guardian
M
O
L
E
C
U
L
A
R
More clues leading to the result MOLECULAR
- . Second cure-all made up of groups of atoms
- Atomic group’s spy meets university lecturer in vehicle
- Instant cure-all, possibly composed of tiny pieces
- Tooth containing unexpected clue regarding small particles
- Plant developed a curl of the smallest chemical bit
- One burrowing dog bites the French in little bits
- ‘Minuscule tooth-filling’ clue set
Buddy explains!

The straight part
We know that "Atomic group’s" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case atomic group’s = molecular.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 MOLECULAR mean?
MOLECULAR noun- Any compound having a specified (range of) molecular weight(s)
- Relating to, or consisting of, or produced by molecules.
- (of an element) Combined with itself and with no other element; elemental.
- Relating to a simple or basic structure or organization.
- Relating to molecular gastronomy.