For your abs to show good muscle definition you need to get to around 4-6% bodyfat. To drop bodyfat you have to cut nearly all fats out of your diet and step up your fat burning routines so you are burning about 1500 calories over your overall intake. What you are looking to do is a difficult fight of balance... At your weight 1% BF loss would drop your weight is 1.5lbs. I am guessing you need to drop your BF about 3-4% which would be a ~5.5lb weight loss and would require you to replace that fat with 5.5lbs of muscle if you wanted to stay even.
Part of this is also genetics though as some muscles in some people are more defined than others...
"A pint of sweat, saves a gallon of blood." - General George Patton