Words spoken privately since ill-omened day (6)
I believe the answer is:
asides
'privately since ill-omened day' is the definition.
I can't judge whether this definition defines the answer.
'words spoken' is the wordplay.
'words' becomes 'aside' (aside is a kind of words).
'spoken' becomes 's' (this might be a standard abbreviation I've not previously seen).
'aside'+'s'='ASIDES'
Can you help me to learn more?
(Other definitions for asides that I've seen before include "throwaway comments" , "Words spoken in undertones" , "Players aren't meant to hear these" , "Remarks not for overhearing" , "Undertones on stage" .)