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