A clausal subject is a clausal syntactic subject of a clause, i.e., the subject is itself a clause.
Xonani tozalashingiz kerak. \n You need to clean the room.
csubj(kerak, tozalashingiz)
Kitobni o’qish kerak. \n You need to read the book.
csubj(kerak, o’qish)
Yaxshiyam tozalabsiz. \n Good that you have cleaned.
csubj(yaxshiyam, tozalabsiz)