Big Kev,
I've not really considered this side-effect of turns before. So as there is no confusion, I am asumming perfect ailerons and so there is no need for rudder to produce a balanced turn.
Because, as you say, the weight is straight down but the upforce component is reduced because of the angle of the lift vector then the aircraft will simply descend vertically, but the aircraft is at an angle so hence the understandable use of the term "sideslip" in this case. Therefore, there is the need to increase the lift vector by such an amount that the upforce component again matches the weight to prevent the sink. This is accomplished by increasing the angle of attack of the wing through the use of the up elevator control.
To answer your question, sideslip in a perfectly executed level turn, where the pilot correctly compensates for reduced upforce with up elevator, would be non-existent. In reality...
In a climbing or descending turn then there must be a small amount of sideslip. In practical terms it is nugatory.
The angle of bank could be so great that it is not possible to compensate with up elevator. That is another matter.