Quietly greeting Greek character (3)
I believe the answer is:
phi
'greek character' is the definition.
(letter from the Greek alphabet)
'quietly greeting' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'greeting' becomes 'hi' (hi is a kind of greeting).
'p'+'hi'='PHI'
(Other definitions for phi that I've seen before include "Letter from the Balkans" , "Foreign character" , "Greek letter; hip (anag.)" , "Greek letter in previous answer" , "classic character" .)