Back fat.....

Mine improved with swimming back stroke and crawl. But it's still a bit far around the armpit area
I'm 46 weigh 10.2lb and 5.4" so I reckon it will never get much better as I'm approaching menopause and already find it tough to maintain weight. I'm size 12 in most shops.

As Auchan said, there is no exercise that will target fat. Losing weight is the only way to combat fat and even then you can't control where you lose it from.

I can promise that no matter what exercise you do you cannot target a specific area for fat loss only muscle gain, toning etc. Your body will lose that fat how and when it likes. Doing some weight lifting might tone the area enough that it's not so noticeable.