The most common and most likely answer to this crossword clue is the 4 plus 3 letter word LAID LOW
I've seen this crossword clue in The Daily Telegraph
L
A
I
D
L
O
W
More clues leading to the result LAID LOW
- Struck by illness, desperately do a will
- Suffering hilly area, one almost entirely dug up
- Stricken, face turning blue
- Stricken boy transfixed by one beastly noise
- Bedridden chap lacking iron receives help
- Learners without financial support — it hurts, making one sick
- Reduced to inactivity, face the other way and look west
- Hid away directive repressing former international uprising
- Six feet under — or heading that way?
- Affected by illness after fifty, help will be falling short
Buddy explains!

The straight part
We know that "Six feet under — or heading that way" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case six feet under — or heading that way = laid low.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.