What do you mean how well do you have to clean it? You have to clean and sanitize it just like you need to for your primary.
Unless introducing spices / fruit or some other addition there is no need to even put it into a secondary. (unless this is a beer that is going into extended aging) Leave it in the primary for three weeks and call it good.
OR - perhaps I am reading your question wrong. Are you planning on pitching on the previous yeast cake? You can pitch right onto it if you like, just sanitize the top / neck of the fermenter and your transfer equipment. This is of course assuming that nothing went wrong with that first batch.
I am asuming you are going to rack what is in your primary fermeter to your secondary and imediatley put another batch directly onto the yeast cake in your primary. You are asking if you have to do anything to clean your primary out before you put your next batch in correct?
The short awnser is you do not have to do anything at all to your primary fermenter as long as you are happy with the results of the original beer in the primary. By that I mean there are no off flavors or aromas and your beer tastes and smells like success.
Also make sure your new batch is similar to what you just took out. You dont want to be putting say a light pale ale on top of a stout yeast cake. you can however go the oposite way with no problems.