Shortly, you'll find it next to London on the map (5)
I believe the answer is:
derry
'map' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for derry that I've seen before include "Port" , "Ulster city" , "'The London - - Air' (mus.)" .)