Scare causing a number to prepare for war (5)
I believe the answer is:
alarm
'scare' is the definition.
(alarming is a kind of scaring)
'a number to prepare for war' is the wordplay.
'number' becomes 'L' (L is an example).
'to prepare for war' becomes 'arm' (as in arming with weapons).
'a'+'l'+'arm'='ALARM'
'causing' acts as a link.
(Other definitions for alarm that I've seen before include "Warning bell" , "Shock; danger signal" , "Device sounding danger" , "Fright" , "Deep concern" .)