Hi Anthony. Sucks about your knife. It would be helpful if you let us know where you are located/where this happened. Also, how did the police happen upon the knife. Unless you are is a severely oppressed situation (UK) I find very strange that the police would see you with a clip on your pocket, detain you and seize your knife as a "gravity knife".
Finally, assuming everything else checks out, did you get a receipt for the knife, a court date, officer's name, incident report or anything of this nature?