Essentially, earl acts nice up North (6)
I believe the answer is:
arctic
'north' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for arctic that I've seen before include "The frozen north" , "It's cold at the top" , "Cold region in the northern hemisphere" , "Extremely cold" , "Northernmost regions" .)