A large division warning of danger (5)
I believe the answer is:
alarm
'warning of danger' is the definition.
(an alarm alerts you to danger)
'a large division' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'division' becomes 'arm' (arm is a kind of division).
'a'+'l'+'arm'='ALARM'
(Other definitions for alarm that I've seen before include "Warning sound/device" , "Frighten, warning" , "Frighten, disturb" , "Noise that wakes" , "Clock that wakes the sleeper" .)