Scare making nearly all prepare for war (5)
I believe the answer is:
alarm
'scare' is the definition.
(alarming is a kind of scaring)
'nearly all prepare for war' is the wordplay.
'nearly all' becomes 'al'.
'prepare for war' becomes 'arm' (as in arming with weapons).
'al'+'arm'='ALARM'
'making' is the link.
(Other definitions for alarm that I've seen before include "Burglar deterrent" , "Wake-up call?" , "Daunt" , "Deep concern" , "A rousing sound in the morning" .)