Blood group negative involves nothing sinister (7)
I believe the answer is:
ominous
'sinister' is the definition.
(synonyms)
'blood group negative involves nothing' is the wordplay.
'blood group' becomes 'O' (O is one of the main blood groups).
'negative' becomes 'minus' (similar in meaning).
'involves' 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 "Threatening, looks bad" , "'Threatening, foreshadowing tragedy (7)'" , "Looking like trouble coming" , "Boding ill" , "Foreboding, menacing" .)