Greek character’s quiet greeting (3)
I believe the answer is:
phi
'greek character's' is the definition.
(letter from the Greek alphabet)
'quiet greeting' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'greeting' becomes 'hi' (hi is a kind of greeting).
'p'+'hi'='PHI'
(Other definitions for phi that I've seen before include "Greek letter; sounds like expression of disgust" , "Greek letter sounding F" , "Greek letter; sounds like expression of outrage" , "Twenty First Greek letter, pronounced as f" , "Greek character" .)