Anything that you have done in the past, paid or unpaid, that lends itself to the requirements of the position. Even if your title was not "Office Clerk" specifically, you can draw on your previous experience from anywhere to detail how it speaks to the job description.

Simply stating "I don't have any." The interviewer has your resume, and if you don't have direct experience, they already know that. You're there, so the interviewer has seen something that they like about you.

"When I was with XX company, I supported several individuals performing varied positions from Accounting to Marketing. I answered and transferred telephone calls, took messages, sorted and delivered mail, scheduled appointments, and received customers and visitors."

"Although I do not have direct Office Clerk experience, when I worked as a front office receptionist (or other) I provided general information to staff, clients, or visitors, created documents in MS Office suite, and filed paper and electronic documents."