Hardly an expert... but let's see...
Clown tang, eh? And he harasses the Foxface? He may have stabbed him with his scalpel(s).
Typically though a stab wound will either look fairly clean, and heal after a few days, or it gets infected, and red.
The photo it looks more brown to me - could be bruising perhaps.
How is the fish behaving otherwise?
Did this just occur suddenly - as in, it wasn't there one day, and poof, it's there?
If it's eating and otherwise behaving normally, I'd keep an eye on it for a day or two. If it worsens or if the fish becomes lethargic, stops eating etc., then intervention might be in order.
If the wound looks like it's infected, then the best option would be to QT and treat... but let me know what's going on based on my questions and we can figure it out from there.
Uncomplicated mechanical injuries in fish usually heal pretty quickly. Infection changes the game.
It could also be a sting from an anemone (if present) or LPS coral but when you mentioned the clown tang, the first thing that came to mind was a scalpel injury.
Jenn