I have a question about inserting information into a quotation from a text.?

I am writing an essay for English cl, and would like to use this quote in my writing: "the usual principle of succession required that the crown p to a collateral of the king, not to heirs of the direct line". Because I am using this quote partially to explain what "collateral" means. I would like to add the definition in there somewhere. However, I am wondering if it is grammatically correct to do it like so: "...to a collateral [one who is descended of a common ancestor, but through different lines] of the king..", or should I simply interrupt the quote, like this?: "...to a collateral" - one who is ...different lines - "of the king..." ? Any appropriate and reliable answers would be accepted, thank you very much !