Computer cover under table (7)
I believe the answer is:
desktop
'computer' is the definition.
(desktop computer is a kind of computer)
'cover under table' is the wordplay.
'cover' becomes 'top' (topping is a kind of covering**).
'under' means one lot of letters go next to another.
'table' becomes 'desk' (desk is a kind of table).
'top' put after 'desk' is 'DESKTOP'.
(Other definitions for desktop that I've seen before include "working area in office" , "Sort of publishing" , "Location of icons" , "Background in computing" , "Screen display on PC" .)