It looks like a wound to me - the main thing you want to avoid is an infection setting in. Make sure your water quality is up to scratch & if not do a water change. Feed her well each day.
The least stressful way to help prevent infection is by doing an IN-TANK hyposalinity treatment - you do this by lowering your salinity to 1.017. This is safe for sensitive invertebrates & corals and won't kill off your natural filter. Additionally, if the clown is easily netted (do not chase) and you have Betadine in the cupboard at home, get a 5ml syringe from the chemist and daily, mix 1 cup of water from the tank with a rice grain amount of Betadine mixed thoroughly. Draw up some of the diluted Betadine mixture into the syringe. Net the clownfish, put your hand around the fish gently so you have good control. Holding the fish over a dish or something, drizzle the wound with the Betadine in the syringe. This way you can target the area without getting it in the fish eyes or over its gills (Betadine can sting...).
Keep doing this until the wound is completely healed. I hope the outcome is good!
Is this the nano you just set up? What are your ammonia readings?