### Clue: 0 plus (-50) plus 500? On reflect­ion, that's the product of a higher power (4,3)
### Answer: OPUSDEI 
 ### Explanation: Definition: t

**O **(0) + **P**(l)**US** (*minus* L (fifty)) + **D** (500) + [*on reflection*] **<=(i.e.)** ("that's")