Notes a difference at the start (5)
I believe the answer is:
onset
'the start' is the definition.
(both can mean the beginning of something)
'notes a difference' is the wordplay.
'a difference' is an anagram indicator.
'notes' is an anagram of 'ONSET'.
'at' acts as a link.
(Other definitions for onset that I've seen before include "Beginning (of something often unpleasant)" , "Appearance of" , "Stone (anag)" , "Beginning; violent attack" , "origin" .)