Sinister order in America enlisting Oscar (7)
I believe the answer is:
ominous
'sinister' is the definition.
(I know that sinister can be written as ominous)
'order in america enlisting oscar' is the wordplay.
'order' becomes 'o' (abbreviation e.g. in British honours OM, OBE etc.).
'in' means one lot of letters goes inside another.
'america enlisting' becomes 'minus' (I can't justify this - if you can you should believe this answer much more).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'minus' going around 'o' is 'minous'.
'o'+'minous'='OMINOUS'
Can you help me to learn more?
(Other definitions for ominous that I've seen before include "Looking like trouble coming" , "Portending bad things" , "Minatory, trouble coming" , "Threatening, not looking good" , "Boding ill" .)