cum hoc: i.e. Clodio.
alium testem: this suggests a doubt of the truth of Pompey's statement. There can in fact be little doubt that Caesar and Pompey understood the designs of Clodius, and tacitly approved of his election to the tribuneship. It was part of their plan to break down the prestige of the senate, and that could be accomplished in no better way than by degrading one of its leaders and discrediting its somewhat autocratic treatment of the Catilinarian conspiracy.
cum . . passus esset: Pompey actually took some part in the proceedings of the comitia curiata when Clodius was adopted; cf. Att. 2.12.1.
fidem recepisse, etc.: 'both Clodius and Appius have given him (Pompey) a promise not to attack me.' Recipio in this sense is colloquial. The full expression is in me recipio.
Appium: Appius Claudius Pulcher, the brother of Clodius, had been Cicero's friend until the quarrel with Clodius occurred. He was in 52 B.C.
Cicero's predecessor as governor of Cilicia. The 13 letters of Bk. 3 ad Fam., are addressed to him.
multa contra : (sc. dixisse): cf. Intr. 95. The verb of saying is most frequently omitted, as here, in reporting the words of another.