They should unless sitting down would be a determent to the effectiveness of a normal person doing the same job. It might be argued that a sitting down clerk is not as effective as a clerk who is able to stand. In this case they would have to fire him. But this is not a strong argument.
Otherwise, yes I would say there is a legal obligation to provide a chair. Having the clerk provide his own chair is not acceptable.
Also I disagree with those that say that the clerk is required to mention his disability prior to being hired, unless the manager specifically asks, and it is true that a sitting clerk is less effective than a standing clerk. That would invite the manager to discriminate against him on the basis of his disability. If the manager asks, then the would be clerk would be obliged to tell the employer of his disability, and the manager would not be able to higher him, since the job requires a standing clerk by premise.
EDIT: also If prolonged standing is not a determent to the job, then it would be illegal for the manager to ask about such a disability.