To clip a grid image to the bounds of a polygon vector file or X and Y coordinate extents open the Grid Utilities plugin from the Gridding menu and choose the Clip option from the Utlities button drop down list.
If the grid being applied to this process is not already loaded into the current session of Encom PA click on the Open button (yellow folder) at the top of the Grid Utilities dialog and browse to it's location. Once the grid is loaded it will be highlighted in the grid list at the top of the dialog.
In the Clipping Type section choose between a Rectangular or Polygonal Clipping Region. For a Rectangular clipping region specify the Minimum and Maximum X and Y values from the entry boxes available. For a Polygonal clipping region click on the Open vector file button (yellow folder) located near the bottom of the dialog.
Note: If a Feature Database is loaded in the current session the Feature database option will also be available for using a polygonal feature object as the clipping region.  
Click the Update button at the bottom of the dialog to preview the clipped grid. Then when satisfied with the output select the Save As... button at the bottom of the dialog and specify a name for the resultant grid image file.