How to get current color buffer pixel value in shader?

I was wondering how to get the current screen pixel value during rendering in a shader?

I need to clear all pixels alpha to zero, and I was thinking that the best way to do this is to keep the current color, and zero the alpha channel.

Thanks in advance

This did not technically answer the question, so I dont think I should answer with this, though it DID solve my problem:

ColorMask A

and render full screen quad with color.alpha = 0. This wont render to the color channels, but will zero the alpha channel.