Fed up eating cheese? Find out what happened (7)
'find out what happened' is the definition.
(to be debriefed is to find out what happened)
'fed up eating cheese?' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'eating' indicates putting letters inside.
'cheese?' becomes 'brie' (Brie is an example).
'def' placed around 'brie' is 'DEBRIEF'.
(Other definitions for debrief that I've seen before include "Get mission statement", "Question (returning spy)", "Be fired (anag.)", "Gather information from returning agent", "Interview after mission (to gather information)".)