Celestial phenomenon stars follow and not sailors (8,6)
I believe the answer is:
northern lights
'celestial phenomenon' is the definition.
(I've seen this before)
'stars follow and not sailors' is the wordplay.
'stars' becomes 'lights'.
'follow' means one lot of letters go next to another.
'and not' becomes 'nor' (I've seen this before).
'sailors' becomes 'the RN' (the Royal Navy).
'nor'+'thern'='northern'
'lights' after 'northern' is 'NORTHERN LIGHTS'.
(Other definitions for northern lights that I've seen before include "overhead display" , "Blackpool illuminations?" , "a must-see for Arctic cruisers?" , "Celestial phenomenon" , "What flickers in the sky" .)