antique stock > ancient trunk (with the pun on "stock")
5 Through discord, and the royal throne forlorn:
forlorn > abandoned, forsaken; lost
6 Thenceforth this realm was into factions rent,
rent > torn
7 While each of Brutus boasted to be born,
each of Brutus boasted to be born > [each pretender to the throne claimed to be descended from Brutus]
8 That in the end was left no monument
That > [So that] monument > monument; also: evidence, hence: trace
9 Of Brutus, nor of Britons' glory ancient.