Overriding cl package symbols

Hello, I have been reading the source code for parenscript and I am keen to find out how you override locked symbols? In the parenscript package there are exports such as "#:defun". I have tried myself to create my own package that exports defun but i get the following error message(using SBCL) "Lock on package COMMON-LISP violated when proclaiming DEFUN as a function" I'd be very interested in finding out how you got around this. Thank you, James Gunn