Line on map made in order to indulge (9)
I believe the answer is:
longitude
'line on map' is the definition.
'longitude' can be an answer for 'map' (thesaurus). I am unsure of the 'line on' bit.
'order to indulge' is the wordplay.
'order' indicates anagramming the letters.
'to'+'indulge'='toindulge'
'toindulge' is an anagram of 'LONGITUDE'.
'made in' is the link.
Can you help me to learn more?
(Other definitions for longitude that I've seen before include "Length" , "Distance in degrees" , "Degree from Greenwich" , "Imaginary circles of distance east or west" , "this helps define our position" .)