Like the heavy curtains, looking rather old (5)
I believe the answer is:
lined
'old' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I can't see?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for lined that I've seen before include "Of face, marked with creases and wrinkles" , "Wrinkled or with layer of material on inside" , "Went along with" , "Covered inner surface" , "Marked by seams or wrinkles" .)