A betrayal that will pack the theatre (4-3)
I believe the answer is:
sell-out
'a betrayal' is the definition.
(I've seen this before)
'will pack the theatre' is the wordplay.
I cannot quite understand how this works, but
'will' could be 'll' (contracted form, as in 'he'll' - 'he will') and 'll' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'that' acts as a link.
Can you help me to learn more?
(Other definitions for sell-out that I've seen before include "Event with no further ticket availability" , "successful show" , "Betrayal" , "When all tickets are booked in advance" .)