Box office buy (6)
I believe the answer is:
ticket
'box office buy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ticket that I've seen before include "Printed receipt that gives holder rights, as to travel" , "Coupon, admission voucher" , "Slip to show payment of entrance fee" , "Summons served for some traffic offences" , "Slip showing payment of fee or fare" .)