Betray the unavailability of further seats? (4,3)
I believe the answer is:
sell out
'betray' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sell out that I've seen before include "Commercial success - total betrayal" , "When all tickets are booked in advance" , "Betray one's principles" , "What popular performers do" , "dispose of everything" .)