Old hand editor left out (7)
I believe the answer is:
omitted
'left out' is the definition.
('omit' can be a synonym of 'leave out')
'old hand editor' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hand' becomes 'mitt' ('mitt' can be a synonym of 'hand').
'editor' becomes 'ed'.
'o'+'mitt'+'ed'='OMITTED'
(Other definitions for omitted that I've seen before include "let go" , "Left out - neglected" , "forgotten" , "not mentioned" , "Musical aptitude" .)