Block put on room key (5,3)
I believe the answer is:
space bar
'key' is the definition.
(space bar is a kind of key)
'block put on room' is the wordplay.
'block' becomes 'bar' (bar is a kind of block).
'put on' means one lot of letters go next to another.
'room' becomes 'space' (I've seen this before**).
'bar' after 'space' is 'SPACE-BAR'.
(Other definitions for space bar that I've seen before include "A keyboard feature" , "Long key on a typewriter" , "Key feature" , "Longest typing key" , "Largest typewriter key" .)