Beginning to exchange notes (5)
I believe the answer is:
onset
'beginning' is the definition.
(I know that onset is a type of start)
'exchange notes' is the wordplay.
'exchange' indicates an anagram.
'notes' is an anagram of 'ONSET'.
'to' acts as a link.
(Other definitions for onset that I've seen before include "Cast stone in attack" , "First signs" , "origin" , "Attack using stone" , "Stone (anag)" .)