The most common and most likely answer to this crossword clue is the 6 letter word PHYLUM
I've seen this crossword clue in The Guardian
P
H
Y
L
U
M
More clues leading to the result PHYLUM
Buddy explains!
The straight part
We know that "Large group of classes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large group of classes = phylum.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 PHYLUM mean?
PHYLUM noun- A rank in the classification of organisms, below kingdom and above class; also called a division, especially in describing plants; a taxon at that rank
- A large division of possibly related languages, or a major language family which is not subordinate to another.