Mother is changing line on map (8)
I believe the answer is:
isotherm
'line on map' is the definition.
(I've seen this before)
'mother is changing' is the wordplay.
'changing' indicates anagramming the letters.
'mother'+'is'='motheris'
'motheris' with letters rearranged gives 'ISOTHERM'.
(Other definitions for isotherm that I've seen before include "Map line connecting places with same temperature" , "Map line of constant temperature" , "Line joining places of equal temperature" , "that's charted to a degree" , "Weather map may show that" .)