Large stone found, what could it be?

If this stone is just thirty feet from a scheduled monument, then it is probably within the zone of notification. This means that any works within the zone are subject to consent from the National Monuments Service.

Technically, cleaning this stone would constitute work within the zone.

There is nothing immediately apparent in your photo to suggest that the stone is culturally significant, although it has been worked. It could have been worked at any period and for a multitude of reasons: a gate post, sill stone, lintel for example.

Probably best to leave it where it is/as it is. It is part of the landscape and cleaning it is unlikely to help or reveal anything.