One can't see they're closed (7)
I believe the answer is:
eyelids
'closed' is the definition.
The definition and answer are not the same part of speech.
'one can't see' is the wordplay.
I cannot really understand how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is found in the answer.
'see' could be 'eye' (I've seen this before) and 'eye' is found in the answer.
This explanation may well be incorrect...
'they're' is the link.
Can you help me to learn more?
(Other definitions for eyelids that I've seen before include "covers for lights?" , "covers for lamps" , "They drop in sleep" .)