The most common and most likely answer to this crossword clue is the 13 letter word CALLISTHENICS
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More clues leading to the result CALLISTHENICS
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 = callisthenics.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 CALLISTHENICS mean?
CALLISTHENICS noun- (in the plural) Gymnastic exercises under the participation of a (surface-wise) multitude of muscles and often minimal equipment (thus, usually bodyweight exercises) with a stress on stamina.
- (in the singular) A system of such exercises.