SANFORD, Fla. — Prosecutors in the second-degree murder trial of George Zimmerman scrambled Tuesday to undo damage to their case by one of their leading witnesses, a Sanford police officer who interviewed the defendant hours after he fatally shot Trayvon Martin. The witness, Officer Chris Serino, testified under cross-examination on late Monday afternoon that Mr. Zimmerman seemed to be telling the truth when he said he had fired his gun in self-defense. The officer’s admission made for a dramatic moment in the trial — and was a clear boon for the defense — but drew no immediate objection from the...