Cosmic event covered in large report (3,4)
I believe the answer is:
big bang
'cosmic event' is the definition.
'covered in large report' is the wordplay.
'covered in' says to put letters next to each other (some letters go on top of or cover others).
'large' becomes 'big' ('big' can be a synonym of 'large').
'report' becomes 'bang' (I've seen this before**).
'big' next to 'bang' is 'BIG BANG'.
(Other definitions for big bang that I've seen before include "Theory of world creation" , "Theoretical first ever moment?" , "Stock Exchange reform" , "Beginning of the universe according to theory" , "Universe's birth?" .)