Depending on the context, a few ways would be to use the Expression Services to reference the combo from your query:
SELECT * FROM SomeTable WHERE SomeField = [Forms]![FormName]![ComboBoxName]
or maybe use the AfterUpdate event of the combo to build a dynamic SQL statement:
Dim strSQL As String
strSQL = "SELECT * FROM SomeTable WHERE SomeField = """ & Me.ComboBoxName & """"
someobject.Recordsource = strSQL