Start making notes (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'making notes' is the wordplay.
'making' indicates anagramming the letters.
'notes' is an anagram of 'ONSET'.
(Other definitions for onset that I've seen before include "Storming" , "Advent" , "First signs" , "Beginning (of something often unpleasant)" , "Start, beginning" .)