A river some tour a lot (4)
I believe the answer is:
ural
'a river' is the definition.
(I've seen this in other clues)
'some tour a lot' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'URAL' can be found hidden inside 'tour a lot'.
(Other definitions for ural that I've seen before include "River of central Russia" , "The - mountains separate Asia and Europe" , "Mountain range between Europe and Asia" , "2,400 km river flowing through Russia and Kazakhstan into the Caspian Sea" , "The .... Mountains divide Europe and Asia" .)