Swiss lake (6)
I believe the answer is:
geneva
'swiss lake' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for geneva that I've seen before include "Avenge in Swiss city" , "Average in Swiss city" , "old Calvinist stronghold" , "International agreement on the treatment of civilians and captured or wounded military personnel in wartime" , "European city" .)