Cryptic crossword clue
Leaning back, a boy exercises a part of the 10  8

The most common and most likely answer to this crossword clue is the 8 letter word AMYGDALA

I've seen this crossword clue in The Guardian
A
M
Y
G
D
A
L
A

Buddy explains!

The straight part
We know that "part of the 10" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of the 10 = amygdala.
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 AMYGDALA mean?

AMYGDALA noun
  1. The region of the brain, located in the medial temporal lobe, believed to play a key role in the emotions, such as fear and pleasure, in both animals and humans.

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