The most common and most likely answer to this crossword clue is the 6 letter word JOGGER
J
O
G
G
E
R
More clues leading to the result JOGGER
Buddy explains!

The straight part
We know that "One exercising" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one exercising = jogger.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 JOGGER mean?
JOGGER noun- A person who jogs (as exercise).
- A tracksuit, particularly the trousers.
- A printing press operator who removes, jogs, and stacks the sheets or signatures of paper.
- (chiefly derogatory) A black person.