Some omen, a certain source of danger (6)
I believe the answer is:
menace
'source of danger' is the definition.
(I've seen this before)
'some omen a certain' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'MENACE' can be found hidden inside 'omen a certain'.
(Other definitions for menace that I've seen before include "Harmful person" , "Threaten with violence" , "Intimidate" , "Threat, danger" , "peril" .)