It's normal for Nathaniel to go by river (7)
I believe the answer is:
natural
'it's normal' is the definition.
(similar in meaning)
'nathaniel to go by river' is the wordplay.
'nathaniel' becomes 'nat' (I've seen this in other clues).
'to go by' means one lot of letters go next to another.
'river' becomes 'ural' (Russian river).
'nat'+'ural'='NATURAL'
'for' acts as a link.
(Other definitions for natural that I've seen before include "Ordinary" , "Inherent" , "Not artificial or man-made" , "Unrefined person" , "Unforced" .)