Hello ka2015.
I'm not the best person to answer this, but I'll give you a method that worked for me.
I bear in mind that:
a. on the main form, you have the field you require as a source field for your filter (in this case, person's ID, person's name, or person's initials field(s), depending on what applies for you).
b. you have the target query (in this case, the clients table or query)
c. on the clients query, there is a field for details of the filter (i.e. the person in 1. above).
Having fulfilled a., b., and c., above:
1. Open your main form in design view. Ensure you have the required fields (as in a. above)
2. From the navigation pane, select your target query (the clients query) and drag it to the main form.
The sub-form wizard dialog opens, asking you to define which fields on your form link to your sub-form, or to choose the provided list
Select the option Define my own. In the drop down given, select the fields on your form which you want to link on your sub-form (i.e. if, Main Form field is Initials, Sub-form field must be Initials - fields having similar data types and data entries).
3. Click Next. Name your sub-form, then Finish.
Save your form.
In the Form view, you will notice that the person on the main form has their respective clients in the sub-form.
To create the drop-down control,
4. Return to Design View
5. On the Design tab, select option Combo Box
6. Draw the combo box on a blank space in your main form.
7. The combo box wizard dialog opens
8. Select the lowest option: "Find a record in my form based on the value I selected in my combo box".
9. Click Next. Choose the Field (e.g. ID or Initials or Person's Name, as in a. and step 2. above)
10. Click Next, up to the last screen for naming your combo box. Click Finish.
You have just completed your look-up form! :)
In the form view, you can use the combo box to select from the list which person on the main form you would like to view.
Below, in the sub-form, the respective person's list of clients is displayed, based on your selection.
All the best, as you accomplish this!