Bog over there, river down here? (6)
I believe the answer is:
canyon
'river down here?' is the definition.
(a canyon might have a river flowing through it)
'bog over there' is the wordplay.
'bog' becomes 'can' (both are slang terms for toilet).
'over there' becomes 'yon' (archaic word meaning 'over there').
'can'+'yon'='CANYON'
(Other definitions for canyon that I've seen before include "Big rift" , "gully" , "A deep gorge with a river, may be Grand" , "deep container for water" , "Deep valley with steep, even Grand, slopes" .)