Clue: Person enjoying liberty heartlessly imprisoning one who rescues cats? (7)
Answer: FIREMAN
Explanation: Definition: who rescues cats?
 "freeman"(person enjoying liberty/one who is not a slave or in bondage) **minus ****its middle letter**(heartlessly) **containing**(imprisoning) I(Roman numeral for "one").
Defn: Answer to the question: "…?"