Flat bottom in display case (6)
I believe the answer is:
dreary
'flat' is the definition.
(thesaurus)
'bottom in display case' is the wordplay.
'bottom' becomes 'rear' ('rear' can be a synonym of 'bottom'**).
'in' indicates putting letters inside.
'case' says to hollow out the word (remove centre letters).
'display' with its middle removed is 'dy'.
'rear' placed into 'dy' is 'DREARY'.
(Other definitions for dreary that I've seen before include "avoiding exciting stuff" , "routine" , "Gloomy, boring" , "Bleak, depressing" , "Dull, tedious" .)