Longest key? (5,3)
I believe the answer is:
space bar
'longest key?' is the definition.
'space bar' can be an answer for 'key?' (I've seen this before). I'm not sure about the 'longest' bit.
'longest key?' is the wordplay.
'longest' becomes 'space' (I can't justify this - if you can you should give a lot more credence to this answer).
'key?' becomes 'bar' (key is a kind of bar).
'space'+'bar'='SPACE-BAR'
Can you help me to learn more?
(Other definitions for space bar that I've seen before include "It is found on a keyboard" , "It's on the bottom row of a keyboard" , "Restaurant at the End of the Universe?" , "Longest typing key" , "provision for thirsty astronauts?" .)