Crossword Genius

Throw away part of pen that’s used up (3)

Ross

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" .)

I've seen this clue in The Times.
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