Country river, one in Russia (5)
I believe the answer is:
rural
'country' is the definition.
(I know that country can be written as rural)
'river one in russia' is the wordplay.
'river' becomes 'r' (abbreviation).
'one' becomes 'a' (a thing is one thing).
'in' means one lot of letters goes inside another.
'russia' becomes 'url' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' going within 'url' is 'ural'.
'r'+'ural'='RURAL'
Can you help me to learn more?
(Other definitions for rural that I've seen before include "Old country fellow" , "Sort of dean" , "Of the country - pastoral" , "living in the countryside" , "Pastoral or agricultural" .)