Old PM lands with English navy, making spectacular display (8,6)
I believe the answer is:
'spectacular display' is the definition.
(display of lights in the sky)
'old pm lands' is the wordplay.
'old pm' becomes 'northern' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lands' becomes 'lights' (lighting is a kind of landing).
'with english navy making' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for northern lights that I've seen before include "Blackpool illuminations?" , "Celestial phenomenon" , "overhead display" , "What flickers in the sky" , "a must-see for Arctic cruisers?" .)