The Severall Answ(er) of Francis Clements Esq(uire) [one of] the Def(endan)ts to the bill of Comp(lainte) of Henry Ellis a minor by
his Prochein amy Francis Ellis Compl(ainant)
This heading shows that the plaintiff is Henry Ellis. As a minor, he has no legal entity and has to be represented by an adult, Francis Ellis, his ‘prochein amy’ (= ‘next friend’, usually an immediate relative). The defendant in the case is Francis Clements, esquire.
For a better understanding of such a court case, you need to start with the plaintiff’s Bill of Complaint, if it survives. This is the defendant’s Answer to that complaint, and therefore not the best place to start.