Following one crosses wrongly holds copper back (9)
I believe the answer is:
successor
'following one' is the definition.
'successor' can be an answer for 'following' (thesaurus). I am unsure of the 'one' bit.
'crosses wrongly holds copper back' is the wordplay.
'wrongly' is an anagram indicator.
'holds' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'back' says the letters should be written in reverse.
'cu' written backwards gives 'uc'.
'crosses' anagrammed gives 'scessor'.
'scessor' enclosing 'uc' is 'SUCCESSOR'.
Can you help me to learn more?
(Other definitions for successor that I've seen before include "Heir" , "Follower (in post)" , "one coming after you" , "One following another" , "One taking over from another" .)