Cause concern? The French invading might (5)
I believe the answer is:
'cause concern?' is the definition.
(to alarm someone is to cause them concern)
'the french invading might' is the wordplay.
'the french' becomes 'la' ('the' in French).
'invading' indicates putting letters inside.
'might' becomes 'arm' (both can mean strength or power).
'la' placed into 'arm' is 'ALARM'.
(Other definitions for alarm that I've seen before include "Security device" , "Warning bell" , "Warning sound or signal" , "Daunt" , "Announcement of danger" .)