Concerned with tense meal? (6)
I believe the answer is:
'meal?' is the definition.
'concerned with tense' is the wordplay.
'concerned with' becomes 're' (re can mean regarding, concerning).
'tense' becomes 'past' (past is a kind of tense).
(Other definitions for repast that I've seen before include "A formal sort of meal", "Something to eat", "A meal concerning time gone by", "Prates about meal", "lunch?".)