Signal in the style of the Marines (5)
I believe the answer is:
alarm
'signal' is the definition.
(I know that alarm is a type of signal)
'in the style of the marines' is the wordplay.
'in the style of' becomes 'a la' (e.g. chicken a la King).
'the marines' becomes 'RM' (abbreviation for 'Royal Marines').
'ala'+'rm'='ALARM'
(Other definitions for alarm that I've seen before include "Warning siren" , "Frighten, disturb" , "Alternative to reveille" , "Panicky arousal" , "Shock; danger signal" .)