Astronomical event showing complete decline (5,7)
I believe the answer is:
total eclipse
'astronomical event' is the definition.
(total eclipse is a type of astronomical event)
'complete decline' is the wordplay.
'complete' becomes 'total' (synonyms).
'decline' becomes 'eclipse' (I've seen this before).
'total'+'eclipse'='TOTAL ECLIPSE'
'showing' acts as a link.
(Other definitions for total eclipse that I've seen before include "Astronomical event" , "Complete obscuration of sun or moon" , "Celestial phenomenon" , "leaves you completely in the dark" , "Period of darkness" .)