The most common and most likely answer to this crossword clue is the 6 letter word RACHIS
R
A
C
H
I
S
Buddy explains!
The straight part
We know that "a line of bones" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a line of bones = rachis.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 RACHIS mean?
RACHIS noun- The spinal column, or the vertebrae of the spine.
- An anatomical shaft or axis in a marine invertebrate.
- The central shaft of a feather.
- The main shaft of either a compound leaf, head of grain, or fern frond.