Distinctive theory about alternative map line (8)
I believe the answer is:
isotherm
'map line' is the definition.
'isotherm' can be an answer for 'line' (isotherm is a kind of line). I am unsure of the 'map' bit.
'distinctive theory about alternative' is the wordplay.
'distinctive theory' becomes 'ism' (I've seen this before).
'about' indicates putting letters inside.
'alternative' becomes 'other' (I've seen this in another clue).
'ism' going around 'other' is 'ISOTHERM'.
Can you help me to learn more?
(Other definitions for isotherm that I've seen before include "that's charted to a degree" , "Map line of constant temperature" , "Weather map may show that" , "Line of equal heat" , "Line joining places of equal temperature" .)