The most common and most likely answer to this crossword clue is the 7 letter word PELOTON
P
E
L
O
T
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N
Buddy explains!

The straight part
We know that "Main cyclist group" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main cyclist group = peloton.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 PELOTON mean?
PELOTON noun- A platoon.
- The main group of riders formed during a cycling road race.