You are ...
... the question "Equine musicians" by Rand al'Thor.
My first is in the kill zone, ...
The thing we are looking for is made up of numbers or digits, because each of the first three lines has a numeral hidden in the first phrase:
My first is in the kill z[one], but not in no man's land.
My second is in a fif[th ree]f knot, but not in rope or string.
My third is in l[one]liness, but not in unison or simultaneity.
(The second line was edited. It had "fourth reef knot" before, which would lead to two numbers.)
My fourth follows a formula, but isn't in a code.
What follows a formula might be a number, too: Formula One.
This should probably also indicate a number. There's nothing after "The fifth", so the number is probably zero. (Rand al'Thor has revealed that this is the right answer in a comment. I had considered it, but didn't know what to do with the whole number.)
Okay, now what?
We've got a five-digit number. This number (or maybe sequence of digits) can mean a lot of things.
Here at Puzzling Stack Exchange, it can identify a question. For example, if you look at the address bar right now, you'll see the 5-digit number 61639. Replacing that with the number we found brings us to "Equine musicians".
That question is supposed to be a hint for [another question] of Rand al'Thor's. I have seen "Equine musicians" not long ago: It was in the list of related links in the sidebar to the "Strange sorceresses" question.