"Child support used to be an item that you can file for deduction before so now if it becomes garnishment then can be treated as itemize of deduction ?"
VolvoGirl is correct. Child support is not tax deductible, and it never was, no matter how you pay it.
"(1) Can I claim as dependent ?"
Paying child support does not make the child your dependent. See the links that VolvoGirl provided to determine whether you can claim the child as a dependent.
"(2) Can I claim only the remaining of my SSN benefit ?"
You have to report the full amount of your Social Security benefits, as shown in box 5 of your Form SSA-1099. You cannot subtract the garnishment. It's the same as if you received the full amount of the benefits, and then wrote a check yourself to pay the child support.
SSN is your Social Security Number, not your benefits.
