Agent sat preparing some food (6)
I believe the answer is:
repast
'some food' is the definition.
(repast is a kind of food)
'agent sat preparing' is the wordplay.
'agent' becomes 'rep' (short for representative).
'preparing' is an anagram indicator (I've seen 'prepared' mean this).
'sat' with letters rearranged gives 'ast'.
'rep'+'ast'='REPAST'
(Other definitions for repast that I've seen before include "Prates about meal" , "Feast, meal" , "lunch?" , "A formal sort of meal" , "Food - tapers (anag)" .)