One playing a small part in Scrooge regularly (3)
I believe the answer is:
cog
'one playing a small part' is the definition.
(as in the phrase 'cog in the machine')
'scrooge regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'scrooge' are 'COG'.
'in' acts as a link.
(Other definitions for cog that I've seen before include "one engaged in revolutionary activity" , "that engages with others in revolution" , "Gear-tooth" , "Projection on a toothed wheel" , "Tooth on gearwheel" .)