We've never cleaned up anything that wasn't farmed at some point in the previous 50 years or so, but we never asked.  Looking at the maps the trees were smaller and the could somewhat, kinda, sort of, if you looked close, make out a field.  Most we did on one farm was around 20 acres that was basically overgrown hay and pasture land with fencerows cutting it up.  Some of it should never have been farmed but most of it is very productive and FSA didn't say a word when we certified it.  We have an owned farm that we plan on clearing part of and some will have sellable timber on it, but it was pasture 40-50 years ago.  Miss a bush hogging a couple years and all of a sudden you need a trackhoe and dozer.