Step 1: Understand the given expression:
“One who runs away from justice.”
Step 2: A person who escapes or avoids the law is called a fugitive.
Step 3: Examine the other options:
Extravagant — spending excessively.
Eccentric — unusual or odd in behavior.
Connoisseur — an expert judge in matters of taste.
Step 4: Therefore, the correct one-word substitution is fugitive.