Spyderco considers the Citadel to be a Restricted Model covered by the Federal Switchblade Act of 1958. If you have the original paperwork that accompanied the Citadel, there should be a Restricted Item Return Form that has you certify that under the Federal Switchblade Act of 1958 and any other applicable federal, state, and local laws, statutes, and regulations that you are legally able to possess the knife (i.e., active duty military, LEO). If you send in a Restricted Item that you are not legally able to possess, Spyderco will not service it nor return it to you.
Basically, if you're a private citizen sending a Restricted Item to Spyderco, you're SOL. You may be able to get service by having a Spyderco dealer send it in on your behalf but YMMV.
My understanding for the reason for all of this, including having a separate Restricted Models discussion forum on here, is that Spyderco, a number of years ago, ran afoul of the Switchblade Act and since then has taken a very hard line interpretation of it.
Finally, pocket clips, if SFO has them in stock, can be ordered from them with usually no problem.