Seen during tour, a long Russian river (4)
I believe the answer is:
ural
'russian river' is the definition.
(I've seen this in other clues)
'seen during tour a long' is the wordplay.
'seen during' indicates the answer is hidden within the clue.
'URAL' is hidden within 'tour a long'.
(Other definitions for ural that I've seen before include "Mountain range in Russia" , "Mountains in Russia" , "River feeding Caspian Sea" , "River of central Russia" , "The .... Mountains divide Europe and Asia" .)