Neglected (7)
'neglected' is the definition.
('omit' can be a synonym of 'neglect')
This is all the clue.
(Other definitions for omitted that I've seen before include "Failed" , "Hairdressing" , "let go" , "forgotten" , "Left out - neglected" .)
(Other definitions for disused that I've seen before include "empty" , "Derelict" , "No longer observed or practised" , "Idle" , "No longer in action" .)