Neglect to mention (4)
I believe the answer is:
omit
'neglect to mention' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omit that I've seen before include "Leave out, do not include (4)" , "Exclude; forget to include" , "Disregard" , "Leave out or overlook" , "Fail (to do something)" .)