The start of everything, a very noisy affair? (3,4)
I believe the answer is:
big bang
'the start of everything' is the definition.
(start of the universe)
'a very noisy affair?' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for big bang that I've seen before include "First ever explosion?" , "end for 1 across 12 [TRADING FLOOR]" , "beginning to matter?" , "Gary -, Br. footballer" , "Supposed origin of universe" .)