The most common and most likely answer to this crossword clue is the 10 letter word MINORITIES
I've seen this crossword clue in The Guardian
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More clues leading to the result MINORITIES
Buddy explains!

The straight part
We know that "Relatively small groups" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case relatively small groups = minorities.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 MINORITIES mean?
MINORITIES noun- The state of being a minor; youth, the period of a person's life prior to reaching adulthood.
- Any subgroup that does not form a numerical majority.
- (used attributively of a party, government, etc.) Empowered by or representing a minority (usually a plurality) of votes cast, legislative seats, etc., rather than an outright majority thereof.
- A member of an ethnic minority.