The top of the map at the time in question (4)
I believe the answer is:
then
'time in question' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'the top of the map at the' is the wordplay.
'the top of the map' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'at' means one lot of letters go next to another (I've seen this in other clues).
'n' put after 'the' is 'THEN'.
Can you help me to learn more?
(Other definitions for then that I've seen before include "Subsequently - previously" , "Afterwards - as a result" , "At that time; next" , "Consequently" , "Once upon a time" .)