Agent sat preparing dinner, say (6)
I believe the answer is:
repast
'dinner say' is the definition.
(I know that dinner is a type of repast)
'agent sat preparing' is the wordplay.
'agent' becomes 'rep' (short for representative).
'preparing' is an anagram indicator (the same letters prepared differently).
'sat' is an anagram of 'ast'.
'rep'+'ast'='REPAST'
(Other definitions for repast that I've seen before include "Prates about meal" , "Food - tapers (anag)" , "Feast, meal" , "lunch?" , "A meal concerning time gone by" .)