Phonosemantic compound. 口 represents the meaning and 十 represents the sound. Originally a variant of 協 (cooperate). Later used as the simplified form of the phonetically similar character 葉 (leaf).
叶 and 十 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.叶 /*l[a]p/, /*[ɢ]ˤep/ 十 /*t.[g]əp/