"Clear" is relative. The amount of background fog will vary according to a
number of factors. The important thing is that the fog is uniform. When you
print these negatives, a good starting point for print exposure will be when
the "clear" areas print as the maximum black your paper will allow.
-Lew

As Lew resonded, developed film exhibits a base fog
(called film base plus fog and notated as fb+f.) It
varies with the film, developer, and format.

If you have a densitometer to measure film density, fb+f
can be anywhere from .03 (almost non existent) to .20/.40
(about 1 stop) for some 35mm films. "Clear" is always
your film base plus whatever chemical fog there is. You
simply print through it.

  I'm not sure how reliable the exposure meter is as a
densitometer. Assuming it is the density works out to Log
1.0, which is quite high for fog level. Fog density for
ISO-100 film typically is rather low. I just measured the
clear borders of some FP-4 4x5 negatives, the fog plus base
density is around Log 0.1, about a tenth of what you
indicate.
  Airport X-ray usually leaves a herringbone pattern on the
film rather than a uniform fog. This sounds more like fog
induced by high temperatures or some impurity in the
developer. I've used old FP-4 and HP-5 in roll film which
showed no unusual fog after being outdated by more than ten
years so I don't think its age fog.
  Normal film borders look almost entirely clear. My old
negatives have a very slight pink tinge, Kodak T-Max looks
completely clear. This FP-4 was developed in D-76 1:1 and
stopped and fixed about as you indicate above except it was
treated in Kodak Hypo Clearing Agent before washing.
  Its possible the film is light struck but that also tends
to leave some sort of pattern.

I don't think you have a problem. The transmittance (T) of the negative
is 9/11  (transmitted / incident power or intensity) or 0.8181
The optical density or absorbance is - log10 (T) where log10 is the
log to the base 10. The value for the optical density (O.D.) of film
base and fog is 0.087, which is typical of the film.

Some fog is expected with development since developers are not perfectly
selective, and emulsion silver halides can show a bit of reduction to
silver during manufacture and storage.

If you are concerned about the fixer, simply refix a section of the
unexposed processed film, treat with hypoclear, wash and dry. Then
compare it to the old section. If you want to determine the contribution
of silver to the fog, fix and unexposed undeveloped sheet.
Alternatively, you can bleach your processed negative with ferricyanide,
fix and wash.

You should NEVER use a cloudy solution or one with precipitates. It is
easier to mix the solutions a day in advance to permit dissolution of
the chemicals, and filter the developer.

In addition to incomplete dissolution, fixers may become cloudy as the
sulfite anion becomes oxidized. The sulfite reacts with elemental sulfur
 to form thiosulfate (hypo) in solution. If the sulfite isn't in
excess, the fixer may sulfurize leading to a precipitate. The sulfur and
sulfides will react with silver halide to "tone" or fog negatives.

If the fixer isn't completely clear, add a tablespoon or so of bisulfite
(or sulfite) to it, and see if it eventually clears. If not, toss it.
Old or defective packaging may have permitted sulfite oxidation.

Also, if you reuse fixer (a good idea), I'd suggest that you use two
bath fixing for archival processing - 10 minutes each for Kodak Fixer.
The details, theory and capacities are archived. Search for my name,
and archival processing in Google groups.