The interaction is indirect, from the grapefruit juice disabling ways your body has of breaking down Clonazepam (and many other drugs). This will allow more clonazepam to act before it is broken down, and as a result it may not only be stronger but also act longer because it is not broken down and eliminated from the body (goes hand in hand) properly.
It is unpredictable how much stronger or longer drugs act so be careful!
I think grapefruit juice may have an effect for a number of days, and it takes a little while to start working - like an hour maybe more.
Make sure you don't get such strong effects - not only anxiety, there is also effect on your memory among other things, it can make you 'black out' and behave strangely / inappropriately... taking too much benzos is no joke, and combining grapefruit juice with benzos makes them stronger but with uncertain factor.