Don't panic, it doesn't look that bad. Use some warm, soapy water and a cloth to clean the wound and surrounding area. Then, apply some antibiotic ointment (with no "caine/cain" ingredients) to the wound. Before you re-introduce the chick, it would be best if you disguised the wound with some sort of wound/disinfectant spray, like Bluekote--otherwise, the other chicks may peck at the red.
I'd watch the chick carefully for the next few days, for any signs of infection (swelling, discharge, etc.), and make sure it doesn't get picked on. However, it should recover fine.