Tries out wickedness first: that's ominous (8)
I believe the answer is:
sinister
'that's ominous' is the definition.
(synonyms)
'tries out wickedness first' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'wickedness' becomes 'sin' (synonyms).
'first' is a charade indicator (letters next to each other).
'tries' is an anagram of 'ister'.
'ister' put after 'sin' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "not right!" , "Suggestive of evil/harm" , "Forbidding" , "Threatening; evil-looking" , "Menacing" .)