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