A climber seen by river valley (6)
I believe the answer is:
ravine
'valley' is the definition.
(I know that ravine is a type of valley)
'a climber seen by river' is the wordplay.
'climber' becomes 'vine' (climbing plant).
'seen by' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'a'+'vine'='avine'
'avine' after 'r' is 'RAVINE'.
(Other definitions for ravine that I've seen before include "Defile" , "Vainer in narrow gorge" , "Deep narrow gorge between mountains" , "Type of gorge" , "'Deep, steep valley (6)'" .)