Much-used pencil (3)
I believe the answer is:
nub
'much-used pencil' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nub that I've seen before include "Crux (of a matter)" , "Crux, gist" , "Small lump - main point" , "Central point of bun" , "'Gist, central idea (3)'" .)