That being said, don't confuse a clean read with a middle grade novel or a children's story. The theme of a clean read can be as mature as any other novel. It's the author's decision as to how the story is narrated that determines whether or not it is a clean read. When it comes to the content of a specific scene, a clean read leaves more up to the reader's imagination and depends less on the author's prose.
As an example, here's a brief excerpt taken from Beneath Claire's House:
I winced, caught my breath, and saw two orderlies wrestle Angry Gary to the floor amid his screams and shouts of profanity.Because I was writing a supernatural thriller targeted at both teens and adults, I left the details of a relatively intense scene up to the reader, who can now play either the G, PG, or R-rated version in their head. If I was writing an 18+ horror novel, I would have taken a completely different approach to the exact same scene.
I don't advocate every novel should be written cleanly. It depends largely on genre and target demographic, and it's up to the author and publisher to market it appropriately.