Quietly welcome Greek character (3)
I believe the answer is:
phi
'greek character' is the definition.
(letter from the Greek alphabet)
'quietly welcome' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'welcome' becomes 'hi' (both are greetings).
'p'+'hi'='PHI'
(Other definitions for phi that I've seen before include "Twenty First Greek letter, pronounced as f" , "Greek letter; sounds like expression of outrage" , "Foreign character" , "Greek letter, equivalent to F" , "Greek letter, spoken as f" .)