Guitarist's fantastic writing harmonies without me (8)
I believe the answer is:
harrison
'guitarist's' is the definition.
(George Harrison, guitarist with The Beatles)
I don't understand the rest of the clue.
(Other definitions for harrison that I've seen before include "George maybe" , "Guitarist, George - actor, Rex" , "-- Birtwistle, composer" , "Member of group" , "Ford possibly" .)