"A"어/아 놓다 is used to describe a situation where you do "A" and leave the resulting state as it is. It also emphasizes the action already being completed.
So, the sentence "옷을 걸어 놓았다" means that you hung the clothe, and left it hanging.
Other examples using this construction are:
"문을 열어 놓았다." You've opened the door, and left it open
"편지를 남겨 놓았다." You've put a letter there, and left it there.
"불을 켜 놓았다." You've turned on the light, and left it on.