Here's what you need to know about crime in the Middle Ages: everything you know about crime rates, throw out the window. You know the bloods? Crypts? Mafia?
F****ing pansies.
The murder rate was astronomical. There was no police force, as such, whatsoever. No even semblance of one until the Renaissance. Town Watches and the like tried to fill some of the gap in some places, but most "policing" was either community vigilance and noble-hired thugs looking out for their master's interests.
People compensated for the lack of institutional security through personal violence. Insults are more than just "words": they're the equivalent of direct assault. Failing to answer an insult is an open invitation for people to kill you and take your things.
Theft is punished by death, instantly, often by those who catch the theif, not waiting for official results. Nobles and local "magistrates" often were rubber stamps for vigilante or community justice. Stealing even small amounts, like grazing on another's field or stealing an apple from a tree, could get you killed on the spot.
Cities had a little more control than the countryside. The ruling house, houses or guilds could keep an eye on everyone, within reason.
In terms of criminal networks, there was a tonne of smuggling. Tolls were everywhere, and so lots of people were involved in evading them, including nobility and monastic orders (salt, grain and some luxury items were the big rackets.)
Gangs were ubiquitous, but the big rule was that everything was community run. Locals looked after their own, but outsiders were always fair game. There was little
Your character will have to decide what social class he caters to.
Is he the local distributions man for the large-scale smuggling in salt, grain and wool etc? He is the guy who knows where to get the moonshine, and can sell you cheap contraband? Does he facilitate the bribes for the local baliff or governor and the court, and arrange for sketchy legal documents, or can he attain a pardon for the right person? Or has he found a way to circumvent the guild rules on the production or sale of certain materials? At the highest level, in an Italian city, can he arrange for the theft of guild secrets, arrange for assasinations, arrange for fake livery or fake letters patent?
The best use for a clerk, sadly, would be for tax. Taxes were by definition crooked, and were often arranged through a tax-farm system: The King, Duke or whatever, contracts out the right to collect taxes to private contractors who cook the books to inflate their profits. However, this isn't criminal, just corrupt, which wasn't considered an abnormal or really criminal thing at the time.
I hope that helps. If you're more specific, maybe I can help you more. What country are we talking about? What region? What social class? What intended benifits?