Masking of specific class from classified image

I have performed RF classification in Sentinel 1A data. Now i have classified map. And now i want to mask any one class from that map. Please help me.

It depends, how you want to mask them.
If you wont to mark them as no-data you can change the valid pixel expression of your band, then the masked pixel will be shown as no-data. Or you can simply add a mask based on an expression via the Mask Manager and display it as a layer on top of your image.

Sir, I have classified the map in four major class i.e. Crop, Settlement, Water and Forest. Now i want to extract the cropland class with its characteristics. i.e. sigma nought value.

So then create a mask with Mask Manager. Afterwards, you can right-click on the image and chose Export Mask Pixels
This will result in a CSV file

See also the general SNAP video tutorials:

Especially the one named Mask Manager