Fountain-pen filler (3)
I believe the answer is:
ink
'fountain-pen filler' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ink that I've seen before include "Liquid used in printing" , "Printer's medium" , "Dark liquid" , "Writing fluid" , "It is ejected by cuttlefish" .)