What is the definition of the phrase "The fix is in", and what is its
accepted usage?

Hello.

The phrase "the fix is in" comes from the world of gambling.

Consider this explanation by author Tom Nadeau:
"The Fix is In. Whenever professional gambling is considered, there is
always the opportunity and the danger of a 'fix.' This means that the
outcome of a sporting event has already been established in advance,
or that the outcome has been strongly influenced in a certain
direction with the help of unscrupulous insiders who intentionally
adjust the flow of the game in one direction or another. This is
illegal, even in locations where gambling itself is officially
sanctioned and accepted. The essence of the 'fix' is to take what
appears to be a contest of uncertain outcome, and to make sure that it
is really not a gamble at all -- for the one holding the bettor's
money. For the organizer of the wager, the outcome is a 'sure thing.'"
From "Understanding Microsoft" by Tom Nadeau:
http://www.os2hq.com/articles/ums31.htm


Although it has its origins in the world of gambling, the phrase "the
fix is in" is commonly applied to all sorts of events (games,
contests, sporting events, trials, political battles or elections)
that have the outward appearance of fairness but actually have been
secretly rigged so that a certain outcome either will definitely
happen or will be more likely to happen.

Examples of accepted usage:

About politics:

"The fix is in," a frustrated McCain said. "It's because every
lobbyist in this for the high-tech community has said, 'Don't do it.'
The one thing folks in Silicon Valley are scared of more than anything
else is that they would lose their precious stock options."
Source: Buffalo Indymedia 
http://buffalo.indymedia.org/front.php3?article_id=2079

"The fix is in boys, Florida Style. We're being played like a cheap
Stradivarius. The GOP Good Ol' Boy machine is running at full
throttle, its cogs and gears grinding away as the Cuban cigars and
expensive Scotch whiskey are passed around back rooms across the
state. It's a stacked deck in a rigged game."
Source: Southknoxbubba.com:
http://www.southknoxbubba.com/skblog/archive_2002_07.html

From the world of sports:

"But the theory is that the fix is in with the referees, especially
after Spain was eliminated by Korea, when at least one good Spanish
goal was called back."
Source: Eteamz.com:
http://www.eteamz.com/sites/C1/

About TV game shows:

"Despite diligent research nobody has yet turned up evidence that the
fix is in on any of the big money quizzes."
Source: Alphane.com:
http://www.alphane.com/moon/PalmTree/bhob.htm

About life in general:

"All I know is that history is not shaped by a series of random
events. The fix is in. Life is as rigged as big-time wrestling, but
with less discernible heroes and villains."
From Southcoasttoday.com:
http://www.southcoasttoday.com/daily/09-96/09-06-96/d01sp125.htm

search strategy: "fix is in," rigged, slang

I hope this helps.

Thank you very much.  Excellent and exhaustive research on the topic.