Employed an American editor (4)
I believe the answer is:
used
'employed' is the definition.
('use' can be a synonym of 'employ')
'an american editor' is the wordplay.
'an american' becomes 'us' (abbreviation for United States).
'editor' becomes 'ed'.
'us'+'ed'='USED'
(Other definitions for used that I've seen before include "no stranger" , "Sued, having been employed" , "No longer pristine" , "Was treating" , "with a previous owner" .)