The most common and most likely answer to this crossword clue is the 6 letter word GRAVEL
I've seen this crossword clue in Financial Times
G
R
A
V
E
L
Buddy explains!
The straight part
We know that "small rock group" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case small rock group = gravel.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 GRAVEL mean?
GRAVEL noun- Small fragments of rock, used for laying on the beds of roads and railways, and as ballast.
- A type or grade of small rocks, differentiated by mineral type, size range, or other characteristics.
- A particle from 2 to 64 mm in diameter, following the Wentworth scale
- Kidney stones; a deposit of small calculous concretions in the kidneys and the urinary or gall bladder; also, the disease of which they are a symptom.
- To apply a layer of gravel to the surface of a road, etc.
- To puzzle or annoy
- To run (as a ship) upon the gravel or beach; to run aground; to cause to stick fast in gravel or sand.
- To check or stop; to embarrass; to perplex.
- To hurt or lame (a horse) by gravel lodged between the shoe and foot.