Crossword Genius

Obsessives who knew murder cases by heart (5)

Ross

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" .)

I've seen this clue in The Guardian.
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