After solving the problem, his mood became unclouded.

unclouded 是由前缀 un-（表示"不、无"）和 cloud（云、阴云）加上后缀 -ed 构成的形容词。cloud 在这里比喻"忧虑、烦恼"，unclouded 字面意思是"没有阴云的"，引申为"无忧无虑的、明朗的"。