Clue: Small rescue vessel, in 1945, lib­erated part of the British Isles (4)
Answer: SARK
Explanation: Definition: in 1945, lib­erated part of the British Isles
S + ARK. Sark is one of the Channel Islands, liberated from Nazi occupation in 1945. (Geographically speaking the Channel Islands are not strictly part of the British Isles)