Clue: Original note omitted from strait- laced prayer-book (10)
Answer: PRIMORDIAL
Explanation: Original note omitted from strait- laced prayer-book (10)
**PRIM** ("straight-laced") + **ORDI**(n)**AL** ("prayer-book", *with* **N**(ote) *omitted*) 
*According to Chambers, an ordinal is an ecclesiastical order book.