Sinister, nothing less, when cipher's included (7)
I believe the answer is:
ominous
'sinister' is the definition.
(I know that sinister can be written as ominous)
'nothing less when cipher's included' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'less when' becomes 'minus' (I can't justify this - if you can you should believe this answer much more).
'cipher' becomes 'o' (old-fashioned term for zero).
'included' means one lot of letters goes inside another.
'minus' going around 'o' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "Portentous" , "Portending bad things" , "Threatening, portending tragedy" , "Threatening, foreshadowing something bad" , "Sinister-looking" .)