The most common and most likely answer to this crossword clue is the 6 letter word FALLER
I've seen this crossword clue in Financial Times
F
A
L
L
E
R
More clues leading to the result FALLER
Buddy explains!

The straight part
We know that "one not completing the course" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one not completing the course = faller.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 FALLER mean?
FALLER noun- One who falls.
- A fruit that falls from the tree, rather than being picked.
- A part which acts by falling, such as a stamp in a fulling mill, or the device in a spinning machine to arrest motion when a thread breaks.