American editor is not new (4)
I believe the answer is:
'is not new' is the definition.
(used or pre-owned as opposed to new)
'american editor' is the wordplay.
'american' becomes 'US' (United States).
'editor' becomes 'ed' (abbreviation).
(Other definitions for used that I've seen before include "Pre-owned" , "Employed - exploited" , "Enjoyed" , "Brought into play" , "Second hand" .)