i'm not sure what the exact definition of sucker is. but that one is just another branch coming off the cutting. normally, i leave them alone to see which one will grow faster. once i know which one will grow better and if there is no issue, i'll clip the other one to make single trunk tree form.
on grown tree with sucker coming off the root ball somewhere, i cut them off as soon as i see them. they do take energy off the main tree. i used to leave them to grow for cuttings, but realized that they take more energy then the main tree and grow much faster.