clean
Colloquial for the state of not using illegal drugs anymore.
Nach einem schweren Entzug ist er jetzt seit drei Jahren clean.
After a difficult withdrawal, he has now been clean for three years.

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Colloquial for the state of not using illegal drugs anymore.
Nach einem schweren Entzug ist er jetzt seit drei Jahren clean.
After a difficult withdrawal, he has now been clean for three years.

The adjective 'clean' is usually used predicatively, meaning after a noun with verbs like 'sein' (to be) or 'bleiben' (to remain). It is not declined. Attributive use before the noun is rare and is more likely in a design context.
Er ist stolz darauf, seit zwei Jahren clean zu sein.
He is proud to have been clean for two years.
'Clean' is a word from English that is used in German only for the specific meanings 'drug-free' or 'minimalist'. It is not a general replacement for 'sauber' (clean).
Ich mag den cleanen Look von skandinavischen Möbeln.
I like the clean look of Scandinavian furniture.
The context makes the meaning clear. If 'clean' refers to a person, it is about abstaining from drugs. If it refers to an object, a room, or a style, the meaning is 'simple' or 'minimalist'.
Es ist ein langer Weg, clean zu werden und zu bleiben.
It is a long road to become and stay clean.
clear
'klar' and 'clean' can both be used for design or style when something seems calm, well organized, and not overloaded. 'klar' mainly emphasizes that something is easy to understand or recognize. 'clean' is more colloquial and, in design, means simple, minimalist, and without unnecessary details. They can be confused because a clean design often also seems clear. In practice: when it is about understandability, 'klar' fits better. When it is about a modern, simple style, 'clean' sounds more natural.

Das Logo wirkt clean, weil es nur aus einfachen Linien besteht.
The logo looks clean because it consists only of simple lines.
Die neue Webseite ist klar aufgebaut, deshalb findet man die Informationen schnell.
The new website is clearly structured, so you can find the information quickly.