Usually, a mob will announce that the tank lost aggro all by himself by hitting the caster in the face.
You can output your targets name in chat macros via %t, e.g.:
There's also %f for your focus targets name but I don't think there's one for your targets target.Code:/raid I'm attacking %t , please assist!
What you could do is this:
Bind it to a key and try. Didn't test it myself yet.Code:/focus targettarget /raid Watch your threat %f, pls keep an eye on your aggro! /clearfocus
It will clear your focus target, make sure you don't need it for anything more important. ^^
There may be some easier workaround using LUA script.