Endless membrane, I concur, is lacking a lacy structure (8)
I believe the answer is:
filigree
'lacy structure' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'endless membrane i concur is lacking a' is the wordplay.
'endless membrane i' becomes 'fili' (I can't justify this - if you can you should give a lot more credence to this answer).
'concur' becomes 'agree' (synonyms).
'is lacking' suggests deleting specific letters.
'agree' with 'a' removed is 'gree'.
'fili'+'gree'='FILIGREE'
Can you help me to learn more?
(Other definitions for filigree that I've seen before include "decorative work" , "Delicate tracery" , "Ornamental work of fine wires" , "Ornamental lacework in gold or other metal" , "Fine ornamental metalwork" .)