In vacuous orgy, male smeared sun oil with menace (9)
I believe the answer is:
ominously
'with menace' is the definition.
(I've seen this before)
'in vacuous orgy male smeared sun oil' is the wordplay.
'in' is an insertion indicator.
'vacuous' means to remove the middle letters.
'male' becomes 'm' (common abbreviation).
'smeared' is an anagram indicator.
'orgy' with its middle removed is 'oy'.
'sun'+'oil'='sunoil'
'sunoil' with letters rearranged gives 'inousl'.
'm'+'inousl'='minousl'
'oy' enclosing 'minousl' is 'OMINOUSLY'.
(Other definitions for ominously that I've seen before include "with menace" , "apprehension" , "Forebodingly" .)