Person not making modest progress, but barely moving! (8)
I believe the answer is:
streaker
'person' is the definition.
(streaker is a kind of person)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for streaker that I've seen before include "Person running without clothes" , "Someone running naked in a public place" , "Naked runner" , "Exhibitionist spectator" , "One who runs naughtily nude in public" .)