Constant revision of maps in cartographer's range (7)
I believe the answer is:
compass
'range' is the definition.
('compass' can be a synonym of 'range')
'constant revision of maps in cartographer's' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'revision of' indicates an anagram.
'in' indicates putting letters inside.
'cartographer' becomes 'os' (Ordnance Survey, UK map-making organisation).
'maps' with letters rearranged gives 'mpas'.
'mpas' going inside 'os' is 'ompass'.
'c'+'ompass'='COMPASS'
(Other definitions for compass that I've seen before include "Navigational aid" , "Drawing instrument" , "Direction finder; scope" , "Range, scope" , "Direction finding instrument - range" .)