When "clémence" is used alone, it seems you can’t attach "une" to it. But when you have some adjective like "étonnante" or "inhabituelle" precede or follow the word "clémence", it seems you need to add "une". I’d like to know the reason for this.
Il a fait preuve de clémence.
Il a fait preuve d'une étonnante clémence.
Il a fait preuve d'une clémence inhabituelle.