|   | 1 |  initial version  | 
The reset command may do what you want.
sage: var('a,b')
(a, b)
sage: x = a+b
sage: x
a + b
sage: reset('x')
sage: x
x
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.