The most common and most likely answer to this crossword clue is the 5 letter word TAGMA
T
A
G
M
A
Buddy explains!
The straight part
We know that "well-defined bit of body" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case well-defined bit of body = tagma.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 TAGMA mean?
TAGMA noun- A specialized grouping of arthropodan segments, such as the head, the thorax, and the abdomen with a common function.
- A military unit of battalion or regiment size, in the Byzantine empire of the 8th - 11th centuries.