Nothing in which to dip quill? Pen's sound! (4)
I believe the answer is:
oink
'pen's sound' is the definition.
(sound from a pig pen)
'nothing in which to dip quill?' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'in which to dip quill?' becomes 'ink' (historical writing implements).
'o'+'ink'='OINK'
(Other definitions for oink that I've seen before include "Pig's grunt" , "What farmer hears" , "Sound made by a pig" , "grunting sound" , "what we hear from Berkshire?" .)