Old sort of sign, nothing in it portending evil (7)
I believe the answer is:
ominous
'portending evil' is the definition.
(I've seen this before)
'old sort of sign nothing in it' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sort of sign' becomes 'minus' (**).
'nothing' becomes 'o' (looks like zero - 0).
'in it' indicates putting letters inside.
'minus' going around 'o' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "'Threatening, foreshadowing tragedy (7)'" , "Portending something bad" , "Portentous" , "'Sinister, baleful (7)'" , "Threatening disaster" .)