If Clen were to show up as a false positive for amphetamines then eating chocolate (phenetlyamine, far closer to amphetamine infact is the backbone of it) before the test would also make you fail...
I hold no responsibility if you loose your job over this, but IMO Clen is probably one of the last things they will be testing for in a workplace drug test as they typically go for common street drugs (Cannabis, MDMA, Amphetamine, Cocaine, Heroin... Then if they are really anal they will test for Prescription Drugs that are abuseable). Unless your career is being an athlete I don't think you have very much to worry about.