Part of musical series, apparently one short? (4)
I believe the answer is:
note
'part of musical series' is the definition.
(musical note)
I cannot understand how the rest of the clue works.
(Other definitions for note that I've seen before include "Heed; record" , "Single tone; short letter" , "Fame - memorandum" , "Memo; piano key" , "Short personal letter" .)