Midday repast (5)
'midday repast' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for luncheon that I've seen before include "midday menu" , "Food" , "part of formal board" , "fare at one?" , "Midday meal" .)
(Another definition for lunch that I've seen is " Midday meal".)