Greek character is quietly retreating (3)
I believe the answer is:
psi
'greek character' is the definition.
(letter from the Greek alphabet)
'is quietly retreating' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'retreating' says the letters should be written in reverse.
'is'+'p'='isp'
'isp' in reverse letter order is 'PSI'.
(Other definitions for psi that I've seen before include "Greek letter; pressure unit" , "Greek letter; pounds per square inch (abbrev.)" , "Pounds per sq. in. (abbr.)" , "Greek character" , "one employed by Socrates?" .)