### Clue: Certain instrument on which sailors introduce Offenbach (8)
### Answer: ABSOLUTE 
 ### Explanation: Definition: Certain
 LUTE(a stringed instrument) **placed below**(on which …, in a down clue) [ ABS(plural of abbrev. for "able bodied seaman") + **first letter of**(introduce) "Offenbach "].
Answer: Not to be doubted or questioned, as in "this is absolute proof".