I found your site through The Query Shark, and would love to get involved in the ER community. It looks like fun.
Aldon “Crash” Dally is a carefree hobo and leader of Chicago’s Shantytown. When teenage runaway Mary Burns stumbles into his realm, suddenly the police are everywhere. IMary has a secret that could destroy a powerful politican, and the two must run hard and fast to save her life, and put an end to the violent tent city shakedowns.
I like the first sentence. It describes him well, though "leader" is a bit weak for an outlaw type. See if you can find a stronger leadership word, because his outsiderness is your hook there.
I'd also suggest giving a sense of the time period. You can put that after "Chicago's", like Chicago's Depression-era shantytown.
That's the sort of insertion that doesn't get a lot of conscious notice, but is absorbed by the reader.
The second line:
When teenage runaway Mary Burns stumbles into his realm, suddenly the police are everywhere.
This is suddenly not about Crash, and you're losing some vitality. You could start with something like "His power is threatened when..." or "His realm is threatened when..." That is, don't lose your focus here.
With the last line, I think you're compressing too much and yet adding too much detail. We don't need to know why her secret is dangerous. What we need to know is why Crash is willing to risk all to save her. That is, when you say that a politician is threatened, that's about the pol's motivation. What's Crash's? That is, in this tight paragraph, you'll create more drama if you keep it unified on Crash.
Great idea! I love the Shantytown angle.