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 they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
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" .)