Well fillers (4)
I believe the answer is:
inks
'well fillers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for inks that I've seen before include "Sink beneath writing fluids" , "Printing fluids" , "Printers' media" , "Liquids used for printing" , "Fluids from cuttlefish" .)