A fixer is a person who carries out assignments for another party or who is skillful at solving problems with antisocial methods. Their value is commensurate with the effectiveness of a society's law enforcement apparatus(es), as well as the potential for profit in abusing social compacts.
Some fixers serve simply as middlemen, having access to various contacts in the criminal underworld and arranging for safe and orderly meetings between clients. Fixers may be hired even when both parties already know each other, serving as nearly-neutral arbiters, mediators or hosts.
A fixer who disposes of bodies or "cleans up" physical evidence of crime is often more specifically called a Cleaner.
In sports, a fixer is someone who makes (usually illegal) arrangements to fix, i.e., manipulate or pre-arrange the outcome of a sporting contest. This is an archaic usage that should not be taken as an implication when used in this setting.