Study rise of evil; topic number one, terror (6,3,6)
I believe the answer is:
dennis the menace
'terror' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'study rise of evil topic number one' is the wordplay.
'study' becomes 'den' (den can mean an office or study).
'rise of' says the letters should be written in reverse (in a down clue, letters go up).
'evil' becomes 'sin' (synonyms).
'topic number' becomes 'themen' (I can't explain this - if you can you should believe this answer much more).
'one' becomes 'ace' (ace has a value of one in some card games).
'sin' back-to-front is 'nis'.
'den'+'nis'+'themen'+'ace'='DENNIS THE MENACE'
Can you help me to learn more?
(Other definitions for dennis the menace that I've seen before include "As children's character" , "Young imp" , "Young tearaway" , "cartoon character" .)