OK, I have a relatively flat and "shapely" stomach, but there is still a layer of fat there that needs to come off. Not completely... I don't want a 6-pack, just more definition, but still feminine.

Thing is, I don't think I should lose any more weight... my body fat percentage is 12% and my lean body mass is 55kg (121lb) and I'm 5'9.

I workout on average 3 times a week, do plenty of crunches and lots of exercises that work the core. But as I understand it, the fat needs to come off for abs to show. So how on earth can I do this and not get dangerously underweight??