Used as a na-adjective to describe something physically clean, morally clean, or free from impurities. Common in compound words like クリーンエネルギー (clean energy).
この部屋はいつもクリーンに保たれている。
This room is always kept clean.
A shift to clean energy is an urgent task.
The new president promised to make the company's management clean and transparent.
This factory discharges clean water.
clean sheet