Small amount subsequently securing run to the Antarctic (8)
I believe the answer is:
'antarctic' is the definition.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for southern that I've seen before include "-- Cross, constellation" , "Of Counties Cork and Kerry, say" , "Austral" , "Antarctica is in this hemisphere" , "Towards the Antarctic" .)