The modifiers of a clause or of any word in a clause are to be construed as part of the clause. In the sentence "Edgar struck Stanley, who promptly fell dead" the subordinate clause who promptly fell dead modifies Stanley, consequently, it is part of the main clause. We have two clauses here, but it is inaccurate to say that they are a main clause, Edgar struck Stanley, and a subordinate clause, who promptly fell dead. Rather we should say that the main clause is Edgar struck Stanley, who promptly fell dead and that the subordinate clause is who promptly fell dead.