This is definitely a mulberry, but it's difficult to tell from the photo whether it's paper, red or white mulberry. Below is a link to a publication that may be of help. Please note the length of the petiole or "leaf stalk" of the paper mulberry on page 5 in contrast to the petioles of the red or white.  If you determine that it's not a paper mulberry, the table on page 6 should help with determining if it's red or white.

https://www.extension.purdue.edu/extmedia/fnr/fnr_237.pdf

Thanks for your response, Phil.

Sadly, I suspect that what I have here is the paper mulberry, but I'll print out the guide you suggested and take it out to compare with my volunteer plant.

(Oh how I wish it were a female red mulberry!)