Pen fill (3)
I believe the answer is:
ink
'pen fill' 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 "Cuttlefish's secretion" , "Liquid in pens" , "It is ejected by cuttlefish" , "Printer's medium" , "Writing medium" .)