How to extract sub-elements via API

gbenigni

I am trying to extract a sub element via API. The field element acquisition has three sub elements info, ac_cron and ac_cron_sort, but I need to show ONLY ac_cron.
I tried in several ways, without success.
At now I am using the following json:
{
"bundles": {
"idno": {},
"acquisition": {}
}
}

But it returns ALL the sub-elements.
Any hint?
Thank you all, folks!
G