Available and unavailable (3)
I believe the answer is:
out
'unavailable' is the definition.
(I've seen this in another clue)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for out that I've seen before include "Old hat" , "In error" , "jury considering verdict" , "Unacceptable" , "Extinguished; abroad" .)