Some idiots ignoring omen (4)
I believe the answer is:
sign
'omen' is the definition.
(I've seen this before)
'some idiots ignoring' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'SIGN' is hidden within 'idiots ignoring'.
(Other definitions for sign that I've seen before include "Ram" , "Put one's name to a document" , "As the star was for 14 across" , "Public notice; Leo, say" , "Augury" .)