A French waterway lacking security? (7)
I believe the answer is:
unsound
'lacking security?' is the definition.
(unsound can mean insecure)
'a french waterway' is the wordplay.
'a french' becomes 'un' ('a' in French).
'waterway' becomes 'sound' (type of body of water).
'un'+'sound'='UNSOUND'
(Other definitions for unsound that I've seen before include "Unsafe or not well-grounded" , "Imperfect or untenable" , "Ill-founded" , "silence?" , "Unreliable, not well" .)