How to understand "cut clean off" in this sentence
In this paragraph:And since many patients lost their thumbs due to trauma, the tissue that surgeons need to reconnect may not be easy to “hook up,” as Rosen phrases it. In some cases where the thumb was cut clean off, all of the necessary vessels are accessible to the surgeon. Burned, blasted, and crushed hands can make things more complicated.
I am confused about here"where the thumb was cut clean off", I mean cut is a "verb" and "clean off" is a verb phrase", and then why they can just use together directly?