The most common and most likely answer to this crossword clue is the 10 letter word THREESOMES
I've seen this crossword clue in Financial Times
T
H
R
E
E
S
O
M
E
S
More clues leading to the result THREESOMES
Buddy explains!

The straight part
We know that "Small groups" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case small groups = threesomes.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 THREESOMES mean?
THREESOMES noun- A group of three people or things.
- An instance of sexual activity involving three people.