Summon up gripping excerpt of heavenly event (8)
I believe the answer is:
ecliptic
'of heavenly event' is the definition.
'summon up gripping excerpt' is the wordplay.
'summon' becomes 'cite' ('cite' can be a synonym of 'summon').
'up' says the letters should be written backwards (in down clue: letters go upwards).
'gripping' is an insertion indicator.
'excerpt' becomes 'clip' (I've seen this before).
'cite' backwards is 'etic'.
'etic' enclosing 'clip' is 'ECLIPTIC'.
(Other definitions for ecliptic that I've seen before include "Apparent heavenly path of the sun's annual motion" , "circle" .)