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?" .)
'formal meal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for repast that I've seen before include "Something to eat" , "Food - tapers (anag)" , "Banquet" , "A formal sort of meal" , "Prates about meal" .)