The most common and most likely answer to this crossword clue is the 6 letter word RIPPER
I've seen this crossword clue in The Daily Telegraph
R
I
P
P
E
R
Buddy explains!
The straight part
We know that "Really good person in Oz/murderer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case really good person in oz/murderer = ripper.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RIPPER mean?
RIPPER noun- Something that rips something else.
- Someone who rips something.
- A legislative bill or act that transfers powers of appointment from the usual holders to a chief executive or a board of officials.
- A murderer who kills and often mutilates victims with a blade or similar sharp weapon.
- A hook-like tool used to tear away ore, rock, etc.
- A person employed to tear away ore, rock, etc. to make a passage for material to be carried to the surface.
- Something that is an excellent example of its kind.
- Software that extracts content from files or storage media.
- A tool or plant used to reduce soil compaction.
- A hot dog deep-fried in oil until the casing bursts.
- A foghorn.
- A hit of a forehand in tennis.
- Very good; excellent; fantastic.
- One who brings fish from the seacoast to markets in inland towns.