### Clue: Personal guide to study old subject (10)
### Answer: CONSCIENCE 
 ### Explanation: Personal guide to study old subject (10)
**CON** ("to study") + **SCIENCE** ("subject") 
*I presume that the "old" in the clue is there to indicate that "science" as a subject has now been replaced by several subsets of "science" such as biology, physics, chemistry etc