Create a module that has the statement:
Public intSelectedClientID as Integer
This gives you a public variable that you can use to carry the value of the Client ID from one form to another.
Set intSelectedClientID to the value selected in the list box you mention.
In the OnOpen property of your main form, put the following code:
If intSelectedClientID > 0 then
me.FilterOn = True
me.Filter = "[ClientID] = " & intSelectedClientID
me.requery
End If
This will display only the selected client. If you want to give the user the option to clear the filter, put a command button on the footer (or header) of your Main Form, with the following code behind the OnClick property:
intSelectedClientID = 0
me.Filter = ""
me.requery
By the way, you could just put the client listbox in the footer (or header) of your main form and add another button (Find Client) that for the OnClick property executes similar code to the first set of code above that filters for the one client.