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This may cause a number to take up weapons (5)

Ross

I believe the answer is:

alarm

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

'this may cause' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?

'a number to take up weapons' is the wordplay.
'number' becomes 'L' (L is an example).
'to take up weapons' becomes 'arm' (I've seen this in another clue).
'a'+'l'+'arm'='ALARM'

Can you help me to learn more?

(Other definitions for alarm that I've seen before include "Warning of danger" , "Tocsin" , "Scare or worry" , "Clock that wakes the sleeper" , "Trepidation" .)

I've seen this clue in The Globe and Mail.
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