The most common and most likely answer to this crossword clue is the 5 letter word LAYER
I've seen this crossword clue in Financial Times
L
A
Y
E
R
More clues leading to the result LAYER
Buddy explains!

The straight part
We know that "A course" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a course = layer.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 LAYER mean?
LAYER noun- A single thickness of some material covering a surface.
- A (usually) horizontal deposit; a stratum.
- One of the items in a hierarchy.
- (by analogy to a stack of transparencies) one in a stack of (initially transparent) drawing surfaces that comprise an image; used to keep elements of an image separate so that they can be modified independently from one another.
- To cut or divide (something) into layers
- To arrange (something) in layers.
- A person who lays things, such as tiles.
- A mature female bird, insect, etc. that is able to lay eggs.
- A hen kept to lay eggs.
- A shoot of a plant, laid underground for growth.