Hi there. I have a couple of questions. How large is the Foxface in comparison to the Naso? How long have you had each of them? How large is your tank?
This is what I do and it may help in your situation- I bought another veggie clip and put it on the opposite side of the tank. The older fish were familiar with the first clip whereas the newer fish started to claim the second clip as "theirs." Eventually all the fish shared both of them. I kept the second clip up but you don't have to once they all finally start to share.
If this doesn't work I would suggest temporarily removing the Foxface to a QT, or somewhere else, to let your Naso adjust without the presence of the Foxface. I'm not talking a long time, just maybe a week or two. Nasos are one of the more calmer fish of the Tangs. Foxfaces, although thought to be docile most of the time, can be aggressive and territorial as you're finding out.
Lastly if the above doesn't work, then I would move around the live rock. I say "lastly" because I personally don't like doing that and it can be a pain in the...
Lastly lastly
D), if the aggression continues I would return one or the other. I doubt it will remain long-term and I'm sure returning one of them will not be necessary. Since the Foxface was there first I'm sure he's just trying to let the Naso know who's "boss" and showing him he has an established territory.
How bad is the aggression? Is the Foxface hurting the Naso? Please keep us updated!