Nothing to write with? Pen's sound! (4)
I believe the answer is:
oink
'pen's sound' is the definition.
'oink' can be an answer for 'sound' (I have seen 'Sound of pig' mean 'oink' so perhaps 'sound' could also mean 'oink'). I am not certain of the 'pen's' bit.
'nothing to write with?' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to' is a charade indicator (letters next to each other).
'write with?' becomes 'ink' (I am not sure about this - if you are sure you should believe this answer much more).
'o'+'ink'='OINK'
Can you help me to learn more?
(Other definitions for oink that I've seen before include "grunting sound" , "Animal sound" , "Sow's sound" , "behave like a pig" , "Porcine sound" .)