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.
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.