I would imagine a 'clean hit' is one made to look like an accident and a 'non-clean' one doesn't take the same precautions?
The police couldn't find a record of the alleged murder victim, so I'm guessing that "redandwhite" and "friendlychemist" were the same person, just playing a con on DPR to get some cash.
If Person A has known motive to kill Person B, in a "non-clean" hit Person A will immediately come under suspicion. If the hit is "clean", anything could have happened and it's a lot harder to establish it as a murder and therefore Person A has a much lower risk of being brought into the case.
Depends how they did it. Shoot someone with an unregistered or stolen rifle from long distance, capture your brass, destroy the rifle and brass. Obviously a murder, if it were done that way. Very little effort involved, and what've the cops really got then?
You'd probably create more evidence trying to make it look like an accident than you'd clear up just by doing it some easier way.
It really depends on where you are. There are vast rural areas in USA, for instance, in which there are many rifles the theft of which wouldn't even be noticed for weeks. Just don't take them during the month before deer season.
My guess is that "clean" includes disposal of the body and other evidence. Like in movies where people call up the "cleaner" after a murder to make the bodies disappear (e.g., "Victor the Cleaner" in Point of No Return).