The most common and most likely answer to this crossword clue is the 6 letter word SIT-UP
I've seen this crossword clue in The Guardian
S
I
T
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Buddy explains!
The straight part
We know that "Exercise" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case exercise = sit-up.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 SIT-UP mean?
SIT-UP noun- An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position.