which引导的定语从句 确实充当成分。在定语从句中，which作为关系代词，可以代替先行词，并且在从句中担当一个成分，这可以是主语或宾语。例如：
1. The book, which I borrowed from the library, is very interesting.
这里，which引导的定语从句“which I borrowed from the library”修饰先行词“the book”，并且在从句中充当宾语。
2. This is the pen which he bought yesterday.
在这个例子中，which引导的定语从句“which he bought yesterday”修饰先行词“the pen”，并且在从句中充当宾语。
3. Football is a game which is liked by most boys.
这里，which引导的定语从句“which is liked by most boys”修饰先行词“football”，并且在从句中充当主语。
综上所述，which引导的定语从句在从句中充当的成分可以是主语或宾语，具体取决于先行词在从句中所扮演的角色以及句子的结构。