Worn-down pencil (4)
(Other definitions for stub that I've seen before include "evidence of smoking, for example" , "Knock (toe); short remnant" , "Knock (e.g. toe)" , "Counterfoil of cheque, receipt etc" , "Extinguish the cigarette on a counterfoil" .)
'worn-down 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 "Central core of bun" , "Central point of issue" , "Centre point or core of a matter" , "Small lump - main point" , "Gist (of a matter)" .)