Observing lack of sound from bell (6)
I believe the answer is:
noting
'observing' is the definition.
('note' can be a synonym of 'observe')
'lack of sound from bell' is the wordplay.
'lack of' becomes 'no' (lack of money is no money).
'sound from bell' becomes 'ting' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'no'+'ting'='NOTING'
Can you help me to learn more?
(Other definitions for noting that I've seen before include "Paying close attention to something" , "Seeing" , "Making brief record as memory aid" , "Recording in writing" , "Observing and jotting down" .)