I think you’re looking into it too deeply. It’s not one particularly word – more a name that identifies who killed St. Clare (a name is a word).
The usual engraved tribute on the General’s sepulcher (or whatever) reads –
Sacred to the Memory of General Sir Arthur St. Clare, Hero and Martyr, who Always Vanquished his Enemies and Always Spared Them, and Was Treacherously Slain by Them At Last. May God in Whom he Trusted both Reward and Revenge him.’
What Father Brown didn’t want to see is –
Sacred to the Memory of General Sir Arthur St. Clare, Hero and Martyr, who Always Vanquished his Enemies and Always Spared Them, and Was Treacherously Slain by Colonel Clancy (or Captain Keith, or President Olivier) At Last. May God in Whom he Trusted both Reward and Revenge him.’
Well, that’s how I read it anyhow!