Cologne's river (5)
I believe the answer is:
rhine
'cologne's river' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rhine that I've seen before include "River flowing from Switzerland, through Germany, to the North Sea" , "Great European river with Lorelei Rock" , "River, rising in Switzerland and flowing into the North Sea" , "Major European river" , "Flower" .)