Hi @coerv
A good question. I just found a similar thread over here:
http://wordpress.stackexchange.com/questions/50331/advanced-custom-fields-wysiwyg-more-tag
You can see that the answer involves getting the value, and applying an excerpt filter to it.
You should be able to use this same code, but obviously change get_field to get_sub_field to find the WYSIWYG value.
Does this help?
Thanks
E