Excessive flex in a driver shaft can contribute to a slice, although it's one of several factors affecting shot direction. The flex of a golf driver influences clubhead release and impacts the clubface angle at impact.
We highly recommend researching club shafts, primarily looking for a golf driver shaft that will match well to your swing speed. We recommend getting a regular shaft for golfers with less than 90 MPH. Anything higher starts to get into the stiff shaft category, which might be appropriate for many golfers.