Using Clio's Bill Themes, you can set your preference to format bills so that the client address shows in an envelope window. This feature is for #10 left-windowed envelopes, and only works for single-pane windowed envelopes.
To enable this feature, go to your Clio Settings panel and click on "Billing" in the Clio Settings column. In the Billing settings screen, click on the "Bill Themes" sub-tab then select the theme that you would like to modify or click the "New Theme" button to create a new theme.
Once you are in editing view for the Bill Theme, select "Page Setup" then "Format", and check the box beside "Make Envelope-Friendly".
Once you have set this up, any bill that you print with this theme will be formatted to fit in #10 left-windowed envelopes.