This happened to Leslie when he got older. It's called a malocclusion, and it happens when the tooth is knocked out of alignment and allowed to overgrow. The only way to fix this is to get the tooth trimmed as soon as possible, because the crooked tooth will make it harder for your rat to eat (and may puncture their gums if it's allowed to grow too much). Most vets should be able to trim the tooth for you, but you'll probably need to schedule regular appointments once every 2-3 weeks to keep it from overgrowing again. Eventually my mother and I were able to trim his tooth ourselves and spare the hour long car drive and hefty bills. Sometimes if the tooth is cut regularly it might re-align itself with time.