"DEFINER" as security mode for a procedure means that the procedure is then executed with the priviliges of the definer of the procedure. In case you create a repository procedure the definer is _SYS_REPO. That would mean that everyone who has the priviliges to execute the procedure, can trigger the execution of the procedure with the priviliges of _SYS_REPO which can be critical.
Use "INVOKER" as security mode to create a procedure which will be executed with the privileges of the invoker. That is possible also on the trial instance. So just write in your procedure "... SQL SECURITY INVOKER ...".