不可以,假设时定语从句,那么先行词是可以代入从句中,
如Who is the woman that is sweeping the floor over there.
先行词是woman,可以代入从句the woman is sweeping the floor over there.
这题的先行词是 idea,代入从句是idea is actually going on in the class room,明显不对,所以不是定语从句,是同位语从句,起补充说明的作用.