### Clue: 5 across imprisoning opponent, not entirely upstanding as 5 across (7)
### Answer: XIMENES 
 ### Explanation: Definition: 5 across
A reversal ('upstanding' in a down light) of SENEMIX, an envelope ('imprisoning') of ENEM[y] ('opponent') cut short ('not entirely') in SIX ('5 across' CARDINAL number).