Herald's quiet errors involved protecting copper (9)
I believe the answer is:
precursor
'herald's' is the definition.
('precursor' can be a synonym of 'herald')
'quiet errors involved protecting copper' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'involved' is an anagram indicator (involved can mean confusing or complex).
'protecting' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'errors' is an anagram of 'rersor'.
'rersor' going around 'cu' is 'recursor'.
'p'+'recursor'='PRECURSOR'
(Other definitions for precursor that I've seen before include "Forerunner, predecessor" , "I was here first" , "Herald" , "Man coming before me" , "Something going in advance of another" .)