Clay soil isn't a problem, as long as it drains. I have my rosemary growing quite happily in my hard, red Alabama clay. I'm careful about watering, of course....but it has done very well. I have several plants.

If you DO decide to amend the soil, be sure to do a very large planting area not just the hole where the plant will go.

Mix in some sand and perlite, dig down about 8 inches deeper than its originally potted in. This should give some dainage. also a mound planting can help too. Rosemary isn't that fussy, as I had it in level 'gravely' soil with minimal drainage and it did just fine.

The best method of amending soil is to add lots of compost to it. It works with heavy clay or sandy soils and anything in between to improve drainage and soil condition. You could toss in a bit of dolomite, too, which helps to break up the clay.

I removed the sod and dumped top soil onto my red clay.
I mixed in compost (cow manure). That was all. The rosemary are actually TOO big now (I planted them 3 years ago) and I constantly prune them back.
They like it dry.....once established, do not water them.

Cow manure, must be composted about a year before use. If added in a fresh state it can damage and kill plants, as well as produce many odd looking weeds.