Consider the following premises. For all y, if y writes a letter, then y is an office clerk. There is at least one y for which if y writes a letter, then y is an office clerk. For all y, if y were an office clerk, then y is writing a letter. Can we infer the following from them? If we can, name the inference rule being used: For all y, if y were an office clerk, then y is writing a letter.
That is one of the premises. Therefore, the answer is yes.