CAD tip # 11608:
Question
1) The recommended way is using a viewport of a paperspace layout - use the MVIEW command to define or change a rectangular viewport - the drawing content will be automatically "trimmed", displayed in a viewport. The model entities will not be really deleted.
2) Another option is to use a true trim. The drawing objects are really modified and/or deleted with this method and some objects crossing the boundary will not be properly trimmed (e.g. texts). This second method can be performed either with the standard command TRIM (select the boundary curve and then window-select the trimmed objects on all sides) or with the Express Tool EXTRIM. To automate trimming and erasing outside objects you can use the LISP utility TrimOuter - see Download.
Both methods shown in a video:

