“当间谍”常见的英文表达是 “be a spy” 或 “act as a spy”。
“be a spy” 强调身份是间谍，例如：He once was a spy for a foreign country. （他曾经是一个外国间谍。）
“act as a spy” 侧重于描述“执行间谍行为、充当间谍角色”这一动作或行为，例如：She was arrested for acting as a spy. （她因充当间谍而被捕。）