The most common and most likely answer to this crossword clue is the 10 letter word HORIZONTAL
I've seen this crossword clue in Independent
H
O
R
I
Z
O
N
T
A
L
More clues leading to the result HORIZONTAL
Buddy explains!

The straight part
We know that "Prone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prone = horizontal.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 HORIZONTAL mean?
HORIZONTAL noun- A horizontal component of a structure
- Horizon
- A Tasmanian shrub or small tree whose main trunk tends to lean over and grow horizontally, Anodopetalum biglandulosum
- Perpendicular to the vertical; parallel to the plane of the horizon; level, flat
- Relating to horizontal markets
- Pertaining to the horizon
- (wine tasting) involving wines of the same vintages but from different wineries
- Of an interval: having the two notes sound successively.