Rattle and bell, often (5)

I believe the answer is:

alarm

I'm a little stuck... Click here to teach me more about this clue!

'bell often' is the definition.
'alarm' can be an answer for 'bell' (I've seen this before). I am not sure about the 'often' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for alarm that I've seen before include "Trepidation" , "Strike fear" , "Burglar deterrent" , "Mild panic" , "Warning siren" .)

I've seen this clue in The Everyman puzzle in The Observer.

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