Apprehension evident in the manner of Royal Marines (5)
I believe the answer is:
alarm
'apprehension' is the definition.
(I've seen this before)
'in the manner of royal marines' is the wordplay.
'in the manner of' becomes 'a la' ('a la' means 'in the manner of', as in 'a la mode').
'royal marines' becomes 'rm'.
'ala'+'rm'='ALARM'
'evident' is the link.
(Other definitions for alarm that I've seen before include "A rousing sound in the morning" , "Warning sound/device" , "Frighten, disturb" , "Security device" , "It signals danger" .)