Alert for datasource instance datapoints based on Instance Description. Currently we cannot create an alert for datasource instance based on Instance description.

To be clear, you're looking to alert if the instance description contains a certain string? If this is the case, theoretically you can make a complex datapoint to evaluate the system.instanceDescription to see if it contains your desired search string. I thought this could be done using pure infix, but it looks like you'd need to make a groovy based complex datapoint to work on the output directly.

Or you're looking to alert if instances with a certain description have a numeric value in a given datapoint over a certain threshold?  If this is the case, @Mike Moniz's suggestion is likely the easiest: clone the DS so that you have one DS monitoring the instances of interest and one monitoring everything else. The DS monitoring the instances of interest would be configured with thresholds that apply to the instances of interest, instead of the thresholds that were cloned.