Someone answered (but now that's deleted for some reason) by saying I should do it like this:
for i=1:numel(aap.acq_details.sessions)
aap.acq_details.sessions(i).name =[];
end
However, this doesn't remove the field. It just sets it to empty array. I want to empty this struct, and put my own values. So I need it completely deleted, blank. It should not even have empty array in it.