Obsessives who knew murder cases by heart (5)
I believe the answer is:
nerds
'obsessives' is the definition.
(nerd can mean someone obsessed with something)
I can't explain the rest of the clue.
(Other definitions for nerds that I've seen before include "Boringly studious people" , "People considered boring because of obsessive hobbies or narrow solitary interests" , "Geeks" , "They can be boringly studious, perhaps lacking social skills" , "Boringly studious types" .)