Fastidious janitor (9)
I believe the answer is:
caretaker
'janitor' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for caretaker that I've seen before include "School worker, perhaps" , "Temporarily in charge" , "One looking after a building" , "Warden" , "exercising temporary control" .)