Quietly agrees it could be some omen (7)
I believe the answer is:
presage
'some omen' is the definition.
(synonyms)
'quietly agrees it could be' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'it could be' indicates an anagram (the same letters could be an anagram).
'agrees' anagrammed gives 'resage'.
'p'+'resage'='PRESAGE'
(Other definitions for presage that I've seen before include "Be a sign or warning of (an event)" , "Foretell" , "Be an advance sign of, portend" , "Be a sign of (something)" , "Forecast, foreshadow" .)