This may cause a number to take up weapons (5)
I believe the answer is:
alarm
'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" .)