Shows no character when depressed (5,3)
I believe the answer is:
space bar
'shows no' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'character when depressed' is the wordplay.
'character' becomes 'space' (space is a kind of character).
'when depressed' becomes 'bar' (I can't explain this - if you can you should believe this answer much more).
'space'+'bar'='SPACE-BAR'
(Other definitions for space bar that I've seen before include "Restaurant at the End of the Universe?" , "provision for thirsty astronauts?" , "A keyboard feature" , "Longest typing key" , "Long key on a typewriter" .)