Box opened by wizard with bishop's long key (5,3)
I believe the answer is:
space bar
'long key' is the definition.
'space bar' can be an answer for 'key' (I've seen this before). I'm unsure of the 'long' bit.
'box opened by wizard with bishop's' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'opened by' indicates putting letters inside (some letters open a gap in others).
'wizard with' becomes 'ace' (I can't justify this - if you can you should give a lot more credence to this answer).
'bishop' becomes 'B' (abbreviation for bishop in chess).
'ace'+'b'='aceb'
'spar' enclosing 'aceb' is 'SPACE-BAR'.
Can you help me to learn more?
(Other definitions for space bar that I've seen before include "key, large one" , "Restaurant at the End of the Universe?" , "Longest typing key" , "provision for thirsty astronauts?" , "Largest typewriter key" .)