Celestial phenomenon is large, in broken pieces (7)
I believe the answer is:
eclipse
'celestial phenomenon' is the definition.
(I've seen this before)
'large in broken pieces' is the wordplay.
'large' becomes 'l'.
'in' means one lot of letters goes inside another.
'broken' indicates anagramming the letters.
'pieces' with letters rearranged gives 'ecipse'.
'l' going within 'ecipse' is 'ECLIPSE'.
'is' acts as a link.
(Other definitions for eclipse that I've seen before include "Darkening of the sun" , "period of darkness" , "Dull time for male duck" , "When one body hides another" , "Momentary disappearance of heavenly body" .)