Threatening nothing less, accepting nothing (7)
I believe the answer is:
ominous
'threatening' is the definition.
('ominous' can be a synonym of 'threatening')
'nothing less accepting nothing' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'less' becomes 'minus' (I've seen this before).
'accepting' is an insertion indicator.
'nothing' becomes 'o' (looks like zero - 0).
'minus' placed around 'o' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "Boding badly, menacing" , "Not looking good, dark clouds ahead" , "'Threatening, foreshadowing tragedy (7)'" , "Threatening, portending tragedy" , "Foreboding, menacing" .)