Line on map is alternative motorway (8)
I believe the answer is:
isotherm
'line on map' is the definition.
(I've seen this before)
'is alternative motorway' is the wordplay.
'alternative' becomes 'other' (I've seen this before).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'is'+'other'+'m'='ISOTHERM'
(Other definitions for isotherm that I've seen before include "Weather map may show that" , "Map line connecting places with same temperature" , "Line joining equally hot spots" , "Contour line of equal temperature" , "Line joining places of equal temperature" .)