Hello
It might help to know what comes before this instruction, if the whole pattern is slip 1 purl wise with yarn in front, p5 and repeat. Or if there is something else before the slip 1.
As it is, assuming this is the only instruction, it is just a slipped stitch with yarn in front, and the ‘sets’ of purl stitches will be the purl 5.
So the repeat, p5, slip 1 p wise with yarn in front, p5, slip 1 p wise with yarn in front… etc.
If this is on the right side of the fabric the yarn strand will be seen sitting across the slipped stitch, to make a pattern. If this is on the wrong side the yarn strand is hidden, the slipped stitch as seen on the right side creates an elegant elongated stitch to produce your pattern.
Vintage patterns are written differently and can take a bit of adapting to. I have read several but not worked one. I think if you just follow strictly what it says it will work out. It may be that the writer doesn’t want to keep repeating “keep the wool to the front of the pin” so is just telling you once that each time the slip stitch purl wise happens you are to do it with yarn in front.
If there are slipped stitches which appear between sets of knit stitches then this piece of information does not refer to it, only to those between sets of purl stitches.
If this isn’t it, or you’re still confused, please do ask again, perhaps with a little more information. It may be a stitch pattern someone recognises.