Variety of top celestial or astronomical event (5,7)
I believe the answer is:
total eclipse
'astronomical event' is the definition.
(total eclipse is a type of astronomical event)
'variety of top celestial' is the wordplay.
'variety of' indicates an anagram.
'top'+'celestial'='topcelestial'
'topcelestial' is an anagram of 'TOTAL ECLIPSE'.
'or' is the link.
(Other definitions for total eclipse that I've seen before include "leaves you completely in the dark" , "Celestial phenomenon" , "Astronomical event" , "Complete obscuration of sun or moon" , "complete blackout" .)