Award dinner involving 500 (5)
I believe the answer is:
medal
'award' is the definition.
(medal is a kind of award)
'dinner involving 500' is the wordplay.
'dinner' becomes 'meal' (I've seen this in other clues).
'involving' means one lot of letters goes inside another.
'500' becomes 'D' (Roman numeral for 500).
'meal' going around 'd' is 'MEDAL'.
(Other definitions for medal that I've seen before include "Award for winning" , "Award for distinction" , "Metal disc awarded as prize, say in Olympics" , "Commemorative piece of metal" , "Olympic award" .)