You a LB?
There are plenty of ways to get your speed up w/o sacrificing muscular size. Plyometrics, sprinting with resistence (parachute, bands, etc.), and sports specific lifts are great examples.
Hang cleans are a great sports specific lift, but most people I've seen do them have terrible form. Here is an example of a power clean with perfect form:
[ame="http://www.youtube.com/watch?v=6TlbDQUWs0s"]YouTube - Power Clean[/ame]
The only difference between this and a hang clean is the starting point, i.e. above or below the knees. It's a hard technique to master but doing them right will give you tremendous starting power off the line. Start light until you have the form down then increase the weight.
Finally, if you run a google search you will find a ton of info on the topic. The first result off a google search of "football speed training" had plenty of articles:
http://www.sport-fitness-advisor.com/football-training.html
EDIT: Just realized that there are advertisments for programs spliced into the site. The info on the site itself is solid but I'd ignore any links to things that you have to purchase.