Brief case in flat (4)
I believe the answer is:
even
'flat' is the definition.
(level)
'brief case' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'case' becomes 'event' (case can mean an event or occurence).
'event' with its final letter taken off is 'EVEN'.
'in' is the link.
(Other definitions for even that I've seen before include "Peaceful" , "Yet; level" , "Not odd; flat" , "Flat - yet" , "Flattened" .)