Printers' media (4)
I believe the answer is:
inks
'printers' media' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for inks that I've seen before include "Printers' media" , "Fluids from cuttlefish" , "Sink beneath writing fluids" , "Printing fluids" , "Printers' mediums" .)