The most common and most likely answer to this crossword clue is the 6 letter word TARSAL
T
A
R
S
A
L
Buddy explains!
The straight part
We know that "describing part of penal structure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case describing part of penal structure = tarsal.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 TARSAL mean?
TARSAL noun- Any of the seven bones of the tarsus.
- Of or relating to the tarsus.