Arctic underground line (8)
I believe the answer is:
northern
I believe this clue is a double definition.
'arctic' is the first definition.
(northern can mean relating to the arctic region)
'underground line' is the second definition.
(line of the London Underground)
(Other definitions for northern that I've seen before include "Not in or from the south" , "Coming from areas closer to the Arctic" , "'... lights, Aurora Borealis (8)'" , "14 13" , "See 9 Across" .)