Is it possible to sue a hiring agency for slander?

Unfortunately, you most likely cannot do anything unless you had a written employment contract which guaranteed you the job--if you did, you could enforce the terms of your rights under the contract with a lawsuit (e.g. for "breach of contract") if necessary. However, in the absence of a contract, employment is employment at will: the employer may terminate someone--or cancel/eliminate a position, or decide to not hire--at any time, for any reason.
Your question says that you wish to pursue action against the "hiring agency." If you had received your communications and offer through a hiring/employment agency but the job was cancelled by the employer they are working for, that would further eliminate your ability to take action: that is because the doctrine discussed below while, to be blunt, rarely works, it does offer at least a possibility of compensation but only if it was the employer itself which made the promises to you. However, if the promises were made by the employment agency, not the employer, "promissory estoppel" is unavailable, since the promises were not made by the one who then eliminated the job.
Promissory estoppel: this is a common law legal doctrine which is not well favored by the courts, since it enforces non-contractual promises in some limited cases (and goes against employment at will), but does sometimes work when ALL the following criteria are met:
1) Someone made you a promise to get you to do something (like of employment, to take a new job);
2) To act on that promise, you had to do something significant to your detriment, like leave an existing job or relocate to a new town/city;
3) AT THE TIME THEY MADE THE PROMISE, THEY KNEW YOU WOULD HAVE TO DO THAT THING TO YOUR DETRIMENT;
4) Knowing the above, they still made the promise;
5) It was *reasonable* to rely on the promise--no warning signs; and
6) They did not honor their promise (which is different from circumstances changing over time; they must not have honored it essentially from the inception).
As you can see though, the promisor must be the one who then does not honor it, but if the agency made the promise and the employer then did not honor, that did not occur.
Also, since courts don't like to enforce non-contractual promises and don't like to overturn employment at will, they are reluctant to use promissory estoppel to enforce promises of jobs, so you cannot at all count on winning--but if conditions are right, you have at least the chance of some compensation.