Polar lights (6)
I believe the answer is:
aurora
'polar lights' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aurora that I've seen before include "-- borealis/australis" , "'... borealis, the northern lights (6)'" , "Polar skies phenomenon" , "Dawn (literary)" , "7 ... borealis, ... lights" .)