Thanks for your response. But am getting "Object doesn't support this property or method" error here. get_inputDomElement.
I got one more solution from my team mate poornima , which is simple and easy..
On client focus just call the following method. OnClientFocus
// This method is used to deselect the text selected by clicking into combobox field
combobox.SelectText = ClickOverride;
Siva kumar KB