Little map on a big map (5)
I believe the answer is:
inset
'little map on a big map' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inset that I've seen before include "Stein (anag)" , "subsidiary diagram" , "Small map within larger; teacher training day" , "Gore" , "Small picture placed within a larger one" .)