Clue: Old instrument promoted after 1100 makes the 23 of 25's landing (7)
Answer: MCMURDO
Explanation: Definition: 23 of 25's landing

**<=(O** (old) + **DRUM** ("instrument")**)** [*promoted*] *after* **MC** (1100 in Roman numerals)
 

 
*McMurdo Sound (23) was where Scott (25) landed on his ill-fated trip to the South Pole.