Box holds Great British key (5,3)
I believe the answer is:
space bar
'key' is the definition.
(I know that space bar is a type of key)
'box holds great british' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'holds' indicates putting letters inside.
'great' becomes 'ace' (ace can informally mean very good).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'ace'+'b'='aceb'
'spar' placed around 'aceb' is 'SPACE-BAR'.
(Other definitions for space bar that I've seen before include "A keyboard feature" , "Longest typing key" , "The biggest typewriter key" , "Largest typewriter key" , "Long key on a typewriter" .)