Instal means: A rare British spelling variant of install, most notably used in the specific sense of paying by instalments. The truncated spelling reflects older British usage where the double-l was sometimes. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.