May be seen as a line on the map (4)
I believe the answer is:
nile
'the map' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'may be seen as a line' is the wordplay.
'line' anagrams to 'NILE'.
However, I am not clear how the anagram is indicated.
'on' is the link.
Can you help me to learn more?
(Other definitions for nile that I've seen before include "River (which is originally blue and white)" , "Green" , "Long river" , "'The longest river, blue and white (4)'" , "River associated with Cleopatra" .)