It not possible for the semen to remain in vagina overnight and then come out as clear discharge in the morning.
The possibilities of clear discharge can be many depending on the other symptoms you have.
First if its your mid cycle time that is ovulation time then clear discharge is normal and happens in every female. It is due to increased level of hormone in ovulation.
The other possibility can a co existing infection whoch would present with discharge, foul smell, itching, redness
or urinary complaints.
I hope I have answered your query.