How the Marines panic? (5)
I believe the answer is:
alarm
'panic?' is the definition.
(I've seen this before)
'how the marines' is the wordplay.
'how' becomes 'ala' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the marines' becomes 'RM' (abbreviation for 'Royal Marines').
'ala'+'rm'='ALARM'
(Other definitions for alarm that I've seen before include "Safety bell or alert signal" , "Announcement of danger" , "Anxious awareness of danger" , "Warning sound/device" , "State of (sudden) fear" .)