Writing implement, a pound (3)
I believe the answer is:
pen
This clue is a double definition.
'writing implement' is the first definition.
(pen is a kind of writing implement)
'a pound' is the second definition.
(I've seen this before)
(Other definitions for pen that I've seen before include "Shut up" , "dash off" , "Writing instrument" , "Female swan" , "See 20" .)