Methuselah repeatedly shelled out for professional solver (6)
I believe the answer is:
sleuth
'solver' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sleuth that I've seen before include "A detective like Sherlock Holmes" , "Detective who follows a trail" , "Olivier/Caine film" , "Private eye" , "tracker" .)