Clue: Prison officer promoted, about to demonstrate the Peter Principle? (5,2)
Answer: SCREW UP
Explanation: Definition: about to demonstrate the Peter Principle?

SCREW ("prison officer") + UP ("promoted").
 
*The Peter Principle postulates that people tend to get promoted to the level of their incompetence (i.e. to a level beyond which they can function well). You only need to look at Westminster or the Capitol for live examples of the principle.