The most common and most likely answer to this crossword clue is the 7 letter word SESTETS
I've seen this crossword clue in The Guardian
S
E
S
T
E
T
S
Buddy explains!
The straight part
We know that "groups of 6" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case groups of 6 = sestets.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 SESTETS mean?
SESTETS noun- A piece of music composed for six voices or six instruments; a sextet or sestuor.
- The last six lines of a sonnet, forming two stanzas of three lines each.