Lady Lavender Hardiness

I have seen "Lady Lavender" elsewhere listed as a tender perennial. Your catalogue lists it as perennial. Could you please clarify?

Lady lavender appears to be slightly less winter hardy than other strains of English lavender. However, since it usually survives the winter at our farm located in zone 5a we list it as perennial. Hardiness designations are meaningless unless the zone is given in which the plant is winter hardy and therefore acts like a perennial rather than an annual. I would guess that the author of the tender perennial designation was in a zone colder than ours.
The nice thing about Lady lavender is that it will flower the first year from seed. Even if it is winter killed, because the owner forgot to mulch it for the winter or the winter was a nasty one with lots of wildly fluctuating temperatures, he will have had a summer of lavender flowers.

Will lavender grow in clay soil that has drainage tiles, in zone 6?

If I say no, someone is bound to prove me wrong! That said, you will more likely have the plant survive the winter if you work some sharp builder's sand into the clay to improve the drainage. Good drainage is especially important in the winter when wet feet kill!