A Continental waterway lacking security (7)
I believe the answer is:
unsound
'lacking security' is the definition.
(unsound can mean insecure)
'a continental waterway' is the wordplay.
'a continental' becomes 'un' (the word 'a' as said on the European continent).
'waterway' becomes 'sound' (type of body of water).
'un'+'sound'='UNSOUND'
(Other definitions for unsound that I've seen before include "Fallacious (argument)" , "Dodgy" , "Faulty" , "silence?" , "Ill-founded" .)