Wrote on complicated series of musical notes (4,3)
I believe the answer is:
tone row
'series of musical notes' is the definition.
'tone row' can be an answer for 'series' (I have seen 'Series of notes' mean 'tone row' so perhaps 'series' could also mean 'tone row'). I'm not sure about the 'of musical notes' bit.
'wrote on complicated' is the wordplay.
'complicated' indicates an anagram.
'wrote'+'on'='wroteon'
'wroteon' is an anagram of 'TONE ROW'.
Can you help me to learn more?
(Other definitions for tone row that I've seen before include "Sequence of twelve notes used in serial music" , "Theme used in serial music" , "Series of notes" , "Basis of serial music" , "note sequence" .)