Old hand editor disregarded (7)
I believe the answer is:
omitted
'disregarded' is the definition.
(I've seen this before)
'old hand editor' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hand' becomes 'mitt' (synonyms).
'editor' becomes 'ed' (abbreviation).
'o'+'mitt'+'ed'='OMITTED'
(Other definitions for omitted that I've seen before include "Leave undone or leave out" , "forgotten" , "Dropped" , "Skipped" , "Musical aptitude" .)