Grunt, seeing old writing material (4)
I believe the answer is:
oink
'grunt' is the definition.
(I've seen this before)
'old writing material' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'writing material' becomes 'ink' (substance for writing with).
'o'+'ink'='OINK'
'seeing' acts as a link.
(Other definitions for oink that I've seen before include "Pig's grunt" , "Sound made by a pig" , "What you get from pen" , "Farmyard sound" , "what we hear from Berkshire?" .)