### Clue: Thing worked out after musical interval determines what you will play! (7,5)
### Answer: TWELFTH NIGHT 
 ### Explanation: Definition: play
  **Anagram of**(worked out) THING ***placed after***(after) TWELFTH(in music, an interval of one octave plus a fifth). 
Answer: The Shakespearean comedic play, aka What You Will, believed to have been written as entertainment for the 12th night, close of the Christmas season. Clever use of the play's alternative title.