Formal meal (8)
(Other definitions for luncheon that I've seen before include "Midday meal" , "formal occasion" , "part of formal board" , "midday menu" , "fare at one?" .)
(Other definitions for dinner that I've seen before include "Supper" , "Main meal of day" , "one making a row?" , "serving in 20 [MESS] ?" , "Repast" .)
'formal meal' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for repast that I've seen before include "Prates about meal" , "Something to eat" , "Banquet" , "lunch?" , "Food - tapers (anag)" .)