Learnt about Northern light (7)
I believe the answer is:
lantern
'light' is the definition.
(lantern is a type of light)
'learnt about northern' is the wordplay.
'about' indicates an anagram.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'learnt' is an anagram of 'lanter'.
'lanter'+'n'='LANTERN'
(Other definitions for lantern that I've seen before include "outside D [DECORATION]" , "Cased lamp" , "Light in a protective casing" , "Enclosed lamp" , "Portable lamp case" .)