Finally..after breaking my head for as while..I came up with 2 solutions...
1) Apply x-panel-ghost
(which is an extjs in build css applied during the drag process) for baseCls or componentCls gave the transparency. But all the other default css for a extjs panel are lost and so we have to manually write them up..
2) The best solution would be to set an image which is made transparent (through photoshop or something) as a background image for the panel header and body and apply it to the cls config of the panel as below:
In panel:
cls: 'transparency'
,
In css file:
.transparency .x-panel-header{
background: transparent url('../image.png') no-repeat center !important;
}
And in the same way, for the body as well..
Hope this helps someone...