You could use the AfterUpdate event procedure of the unbound combo to set
the RecordSource of the form to a SQL string that contains just that one
record.
Example:
Private Sub cboFilterClientID_AfterUpdate
Dim strSql As String
If Not IsNull(Me.cboFilterClientID Then
If Me.Dirty Then Me.Dirty = False 'Save first.
strSql = "SELECT * FROM tblClient WHERE ClientID = " &
Me.cboFilterClientID & ";"
Me.RecordSource = strSql
End If
End Sub