It disappears when filled (7)
I believe the answer is:
vacancy
'it disappears when filled' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for vacancy that I've seen before include "Unoccupied post" , "Lack of consideration" , "Unoccupied situation" , "An opening, job or apartment" , "Unoccupied state, job or apartment" .)