Advance booking condition (11)
I believe the answer is:
reservation
I believe this clue is a double definition.
'advance booking' is the first definition.
(eg a restaurant reservation)
'condition' is the second definition.
(thesaurus)
(Other definitions for reservation that I've seen before include "Booking - qualification" , "Expression of doubt (about a claim)" , "Putting aside" , "Qualification, proviso" , "misgiving" .)