Important fringe event starting time? (3,4)
I believe the answer is:
big bang
'event starting time?' is the definition.
(cosmic event at the beginning of time)
'important fringe' is the wordplay.
'important' becomes 'big' ('big' can be similar in meaning to 'important').
'fringe' becomes 'bang' (I've seen this before).
'big'+'bang'='BIG BANG'
(Other definitions for big bang that I've seen before include "Universe's birth?" , "end for 1 across 12 [TRADING FLOOR]" , "Our origins in theory" , "Theoretical origin of the universe" , "Gary -, Br. footballer" .)