### Clue: Zealous apostle penning a choral song (8)
### Answer: STASIMON 
 ### Explanation: Definition: choral song
An envelope ('penning') of 'a' in ST. SIMON. Not Simon Peter, but another apostle Simon the Zealot, mentioned in Luke 6:15 and Acts 1:13. Stasimon is a choral song in Greek drama.