Plain brown box on top (7)
I believe the answer is:
spartan
'plain' is the definition.
(thesaurus)
'brown box on top' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'box' becomes 'spar' (sparring is a kind of boxing).
'on top' means one lot of letters go next to another (down clues only).
'tan' after 'spar' is 'SPARTAN'.
(Other definitions for spartan that I've seen before include "Resolute in the face of pain -- austere" , "Indifferent to comfort" , "Disciplined person" , "Bleak - rigorous - austere" , "Austere - like Lysander?" .)