Plain brown box on top (7)
I believe the answer is:
Here is my best explanation:
'plain' is the definition.
'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 "Ascetic - ancient Greek", "Of conditions, simple and harsh as for ancient Greeks", "Austere - like Lysander?", "Lacking comfort or luxury", "Disciplined person", "Very strict - austere", "austere old Greek", "Resolute in the face of pain -- austere", "Austere, lacking luxury" and "toughly-disciplined type".)