Kent river is European sea route! (6)
I believe the answer is:
medway
'kent river' is the definition.
'medway' can be an answer for 'river' (river of south-east England). I'm not certain of the 'kent' bit.
'european sea route' is the wordplay.
'european sea' becomes 'med' (short for Mediterranean. I am not sure about the 'european' bit.).
'route' becomes 'way' (way is a kind of route).
'med'+'way'='MEDWAY'
'is' is the link.
Can you help me to learn more?
(Other definitions for medway that I've seen before include "River at Maidstone" , "River of Kent" .)