A quiet boy eats one special hors d'oeuvre (9)
I believe the answer is:
appetiser
'hors d'oeuvre' is the definition.
(I've seen this before)
'a quiet boy eats one special' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'boy' becomes 'peter' (**).
'eats' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'special' becomes 's' (common abbreviation as in Special Air Service).
'i'+'s'='is'
'peter' going around 'is' is 'petiser'.
'a'+'p'+'petiser'='APPETISER'
(Other definitions for appetiser that I've seen before include "Aperitif" , "It's served before a meal" , "Antipasto" , "dim sum?" , "Small starter dish" .)