Fluid in a pen (3)
I believe the answer is:
ink
'fluid in a pen' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ink that I've seen before include "Writing fluid" , "Cuttlefish's secretion" , "See 9" , "Liquid in pens" , "It is ejected by cuttlefish" .)