Throw away part of pen that’s used up (3)
I believe the answer is:
bin
'throw away' is the definition.
(put something in a bin)
'part of pen that's used up' is the wordplay.
'part of pen' becomes 'nib' (I've seen this before).
'that's used up' says the letters should be written in reverse (in a down clue, letters go up).
'nib' in reverse letter order is 'BIN'.
(Other definitions for bin that I've seen before include "Domestic receptacle for rubbish" , "Storage container" , "Discard" , "Common receptacle for household waste" , "Storage receptacle" .)