In addition to being a
red wine, claret is an old
boxing announcer's
euphemism for
blood. In the
good ol' days before
televised fights, one had to turn on the radio and listen to the
play-by-play without being able to see the boxers
having at it. In order to keep things
family friendly, apparently, when a boxer was
bopped in nose one too many times and started to bleed, the announcer would
proclaim something to the effect of, "Look out, folks, the claret has begun to flow!"
Sources:
Stephen King, Dreamcatcher
(who says you can't read horror novels for their educational merit?)
http://www.thedailysports.com/extras/boxing/boxingextras/singlepage/blastfromthepast.htm
(note the commentary from the second round: "...and Gulley put in another dreadful facer, which made the claret fly in all directions...")